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This is the expected behavior of tatoeba.org.
See...
https://en.wiki.tatoeba.org/art...tions-in-grey?

What do you mean by the user activity chart?
Could you add the URL?

> I realize that I didn't describe how to find active contributors ...
One way to find members who have recently contributed is to go to the most-recent "Native Speakers with Contributions" stats.
http://tatoeba.ueuo.com/stats-2020-08-22.html
Stats - 2020-08-22 - Native Speakers with Contributions
Click the "since Last Update" column head, twice to sort the list in order of who has contributed the most sentences in the past 4 weeks.
Or, click the "Language" column to sort the list by the native languages of members, then scroll down to the language you are interested in, and look for members who have contributed since the last update of these stats.
I try to update these stats at least every 4 weeks.

This (new?) one is an interesting idea.
Sentences translated by the largest number of members
https://tatoeba.j-langtools.com...sentences.html

I think a simple "no frills" forum built on CakePHP and integrated into tatoeba.org, so usernames are the same, would be better than moving offsite and would likely be better than the Wall.
A forum wouldn't have to have a full set of features, at least at first. We could have several categories for topics.
1. Questions about how to use tatoeba.org.
2. Discussions about possible improvements.
3. Announcements and progress reports.
etc.
I think the Tatoeba Project has outgrown the Wall for discussions. However, perhaps the Wall could be preserved for TRANG to make official announcements, similar to idea number 3 above.

TRANG asks corpus maintainers not to edit sentences until at least 14 days after a comment goes unanswered, so if there is no comment, all a corpus maintainer can do is to add a comment if there is only a tag and then wait 2 weeks.

This is a project that does somethings like what you are suggesting.
https://commonvoice.mozilla.org/en
You can listen to some of their sentences to get an idea of what the sound quality of the files are like. You can also try recording a few sentences.

We have a new German voice.
Mramosch has contributed 4,390 audio files.
https://tatoeba.org/eng/sentenc...how/168618/und
On August 14, this was up to 6,586 audio files.

I know it's not exactly what you want, but I quickly threw together some pages with 4-word, 5-word, 6-word, 7-word & 8-word English sentences with audio.
http://study.aitech.ac.jp/sente...audio/x-words/
This is based on some programming that I'd already done, so some of the text on the pages may not be fine-tuned for this purpose.

* is it possible to search the Wall?
Not really, However, if you download the weekly export of Wall posts, you can search them offline if you know how to.
* When looking at a sentence, can one see all the tags that have been applied to it?
All tags on a given sentence are shown on the right side of the sentence's page.
* I'm generally interested in longer sentences
You can use the advanced search to find the longest sentences, though it's not possible to easily find sentences with "At least/At most X words."
Here is a pre-filled in advanced search form set up to find the French sentences with the most words that also have English translations. Just go here and input a French word or phrase, or skip inputting a French word and click the "Search" button to find the longest French sentences.
https://tatoeba.org/eng/sentenc...rt_reverse=yes

I don't know if it is possible, but would something like the following work for you if this could be done?
In the advanced search have either both or one of these possibilities.
1. When the results are sorted by the number of words, have the option to only display sentences with X or more words. For example, 5 or more words, 6 or more words, 7 or more words, ...
2. No matter how results are sorted, have the option to skip the first so many results. For example, 500, 1000, 1500, 2000, 2500, ...

We have a new Japanese voice.
Lowteq has contributed 123 audio files.
https://tatoeba.org/eng/sentenc...how/168586/und

** Stats - 2020-07-25 - Native Speakers with Contributions **
http://tatoeba.ueuo.com/stats-2020-07-25.html
Find out who the native speakers are and get links to their sentences.

[#735187] "Tatoeba" in Japanese means "for example". (Eldad)

I didn't notice those 3 dots. I wonder if others will.
1. https://imgur.com/a/1UNnGnu
They don't show on my iPod screen.
2. They didn't show on my desktop screen either. Of course, that's my fault, since I use Stylish to change colors so that tatoeba.org is easily readable for me.
I wish tatoeba.org would consider following some accessibility guidelines to help those who have trouble with low contrast colors.
https://github.com/Tatoeba/tatoeba2/issues/1164
I know it's hard for those who don't have this kind of vision problem to notice things like this.

We have a new Polish voice.
Farmersa has contributed 223 audio files.
https://tatoeba.org/eng/sentenc...how/166739/und

Usually with responsive webpages, what's on the right goes underneath what's on the left on smaller screens.
The way you have it, what's on the right seems to totally disappear.
Here are some screenshots.
New Responsive Search Results on an iPod Touch
https://imgur.com/a/1UNnGnu
New Responsive Search Results on a DeskTop (MacOS Chrome)
https://imgur.com/a/J4ElyF5

I assume that the whole document should be looked at and not just this one section. It sort of looks like someone used machine translation for this.
These phrases seem strange to me and unclear.
"Certain sentences, particularly in audio form,"
"Certain sentences, including those in audio form, "
It's only the audio files themselves and not the sentences, in my understanding.
Maybe you should send the corrections directly to TRANG, so she can update that page.

The following from the "terms of use" is not very clear.
Perhaps "phrases" should be "sentences." There is some other strange language usage, too. Maybe a native English speaker who knows French could help translate the original French page into English.
FROM: https://tatoeba.org/eng/terms_of_use
We are not generally opposed to using our content for commercial purposes. However, this choice depends primarily on contributors. Certain phrases, in particular audio, may be contributed with a non-marketing condition, and therefore will not have to be marketed.
We are not generally opposed to the use of the phrases contributed to our site entailing their modification, whether it is a modification of their content or of their sharing conditions. However, this choice depends primarily on contributors. Certain phrases, including audio, cannot be modified: those contributed with a condition of non-modification (eg. Creative Commons ND: No-Derivative) or a condition of sharing under the same conditions (for example the Creative CommonsSA: 4Share-Alike)

493,678 English Sentences with Audio from the Tatoeba Project
http://study.aitech.ac.jp/sentences/ck/audio/
This is something that I put together for my own students. Perhaps some of you may find it interesting.
The first 36,196 sentences have Japanese translations by native Japanese speakers, followed by 457,482 sentences without.
I updated this today.
On my last update three years ago, on July 8, 2017, there were 295,424 audio files.