Internet is more and more shaping like human life, because well it only exist if humans contribute. When internet was young it acted like a teenager: want to be anonymous and talk crap, spam, troll, now this is like a 20years old growing up man -- wants to put his real identity upfront, upload real pictures, organize and connect with real-life friends, etc.
Facebook lacks next layer, next growing up stage, that is everyone has multiple identities and act accordingly to situation: work, home, friends, family, lovers, etc. This is pretty much what you said already -- I think Facebook failed with "groups" -- they should put more effort into literally enforcing users to organize everyone into real-life "groups", so once this is done you can give options to modify feeds from each person. Now, my wall is equally crapped by people I dont want to hear everyday, and my closest family. Its a mess; like well-build Geocites with nice logo on the top...
IF Facebook wont put more focus on enforcing people to get into groups, I agree that sooner or later they will start declining. I dont really "buy" that whole "everyone is in so I am not going anywhere" point; looks at MySpace -- everyone was there. Fast forward 3 years later everyone is somewhere else. The point is, once a website goes viral, friend of friends and their friends start joining, you won't stop migration of users from one website to another -- but obviously first islander need to see benefits and reasons to move on. Once this start, it will NOT be cool anymore to have a Facebook profile. I cant wait for these days.. not really that I wish bad to someone -- but I want to see how CEO of Facebook will act to save his sinking ship. Oh, I also don't buy the fact that "entire website is using facebook features, such as "like" and forum etc". Trust me, once a new network kicks in, it is obviously will be #1 goal of every webmaster and administrator to attach a new&cool thing to their website. You will see that new widget everywhere within weeks, if not days!!