We know in two ways. First, people have told us via GitHub/IRC/e-mail/etc. and Shirts For Stories page (http://www.rethinkdb.com/community/shirts-for-stories/).

Second, the administration UI checks for version updates, which gives us some information about how many developers are using RethinkDB and how long they stuck around.

There is no perfect way to know because there are many sources of imprecise information, but we're quite confident about the overall conclusions of the data.

> I hope we'll see soon more official drivers...

We've been trying to keep the surface area of the project low for now. Which specific drivers are you interested in? (we probably won't be able to do anything about it for the next ~6 months or so, but having the info helps enormously)