"but I have no idea how it compares to other betas."
That A51 stats compare well with other sites that entered Private Beta in recent times (which is not very many sites: Matter Modeling and Drones were the only two other sites to entire Private Beta in the last 22 months).
Our stats are as follows, after 7 days and a few hours:
Avid users: 33
Total users: 1009
Answer ratio: 1.7
After around 7 days and a few hours, Drones had the following stats:
Questions/day: 21.5 (↑ a lot more than Cardano)
Answered: 99% (↑ a lot more than Cardano, but we were at 99% recently too)
Avid users: 32 (↑ much better than Cardano as a ratio of total users)
Total users: 413 (⬇️not as many as Cardano)
Answer ratio: 1.9 (↑ more than Cardano)
Visits/day: 215 (⬇️not as many as Cardano)
After around 7 days and a few hours, Matter Modeling had the following:
Questions/day: 16 (↑ more than Cardano)
Answered: 70% (⬇️ not as much as Cardano)
Avid users: 35 (↑ much better than Cardano as a ratio of total users*)
Total users: 321 (⬇️ not as many as Cardano)
Answer ratio: 1.5 (⬇️ not as much as Cardano)
Visits/day: 342 (⬇️ not as much as Cardano)
But don't look too much into questions/day at this very early stage: Drones is now at 0.6 and has been under 1 question/day for most of the year, whereas MM is now at 2.8 and has been larger than 2 questions/day for most of the year.
The part that's most concerning is the lack of VOTING.
Only one of the top three most active users is in the top three for voting, and only 50% of the top 10 most active users are in the list of people who have voted at least 10 times.
In fact there's only 25 users that have voted at least 10 times, which is in my opinion abysmally low for a site with 33 "avid" users.
When users spend a lot of time writing a well-formed question or answer and it gets 0 votes, or only a few, it is rare to see them come back.
It's easy to underestimate how many hours people might spend on writing even a 2 paragraph post that has a couple hyperlinks to external references that the user found. Let me quote user gIS from Quantum Computing SE who wrote this on Meta:
"I would just like to remark that upvoting, in my opinion, should not
be meant as some sort of "reward" to be given to exceptional answers
(which is what bounties are for).
Personally, I ask myself a very simple question to decide whether I
should upvote a question or answer: do I believe that this
question/answer should be on this site? It's not really about the
quality of the post, but more about indicating whether I think that
this post is a good fit for the site. How good the post actually is
will still be reflected in the upvotes, because a more useful post
will be found by more people which will therefore hopefully vote it up
Consequently, I believe that upvotes should be given generously and
without too much overthinking. On a practical level, this also
encourages participation to the site, as many people like to see that
their post was appreciated by others, and will therefore be more
likely to stick around more when this happens."