No idea about the app, but the service itself is hugely popular. From what I can tell, there's a huge iceberg of hidden group activity that we don't see.
Interesting in this context that they killed the groups app so as to bring people back into the main app.
Makes me wonder if the real reason is that so many people got sick of "normal" Facebook when all they wanted is to use groups that engagement suffered and they needed to bring everybody back.