If I've understood correctly and looking at the mailing lists, libav is far more active but ffmpeg basically utilizes libav as a kind of upstream, merging from there to itself (on a daily basis was mentioned by the ffmpeg developer at some point IIRC). ffmpeg has also sped up on version number front.
It's not only Debian using libav, but also Ubuntu, so basically a majority of Linux users are using libav. Fedora and openSUSE on the other hand do not include either one because of patent encumberment issues, instead they're only installable from third party repositories. The 3rd party repositories were ffmpeg oriented last I checked.
From what I've also understood, the developer groups are still as separate as in the beginning. It would be nice if LWN wrote an article of the split, but I believe it's very hard topic to handle (really) properly. Almost everything about it is full of IIRC:s, AFAIK:s, guesses and personal e-mails. Some things could be said with all fairness of personalities clashing, but still.