Having played around with diaspora* (awful name), I don't think it can replace Facebook, unfortunately. Like Twitter it doesn't rely on people's real names nor are there good options for chat or making private or public groups or events. However, it could potentially be a good replacement for twitter. We need a good decentralized social network that has good design all around, code, gui, ux etc. which is hard to find atm. Ello is also quite nice, but not free, open and decentralized as diaspora which is a deal breaker for me when discussing new social media services.
Btw, could anyone enlighten me on why jabber (xmmp?) never took off in any major way? It seems nice?