I know what they say about the plural of antidote, but I've personally noticed the same. I use it less, and when I do use it, I notice the people I care about on facebook are using it less too. Everything you say mirrors the response to facebook I've been seeing. Its no longer a toy, its just that thing you have and slowly working towards being a burden.
I think a post I saw about facebook email says it the best: "Facebook, you're giving me email? You're now one ugly paintjob away from being aol". Its initially funny, but the parallels you can draw between AOL and Facebook are huge and I wouldn't be surprised if facebook followed a similar trajectory.