I like the idea of GNU Social, but the Twitter-like frontend, Qvitter, is slow and feels clunky. And last time I checked, it still hadn't solved its "spooling problem": offline nodes won't receive updates and won't catch up when they're back online. A post you delete from your timeline might end up still visible on a node if it was offline when you deleted it.
I'm sort of tempted to try writing a UUCP- or NNTP-based GNU Social clone now.