Doing a quick unscientific poll of a few of my peers, usage of Facebook has changed dramatically since their first 'Facebook experience'. To generalize greatly, the typical user joined Facebook because everyone else was, spent hours and days finding and linking to a few old friends, went through a frantic stage of uploading profiles and pictures, checked Facebook 5 times daily, then started to get annoyed with old distant acquaintances coming out of the woodwork, joined a bunch of silly groups with long names, added 20+ applications, were bitten by a vampire, couldn't figure out what 'poke' really did and why, and then over night they had enough - some are calling this Facebook fatigue.
They now check it every few days, any more and they feel like they are stalkers.
With that said, a very few signed up and never went any further, and an equal few now use it as a substitution for email.
I will become more interested when there are metrics on how users interact and use these social applications over time. It's no surprise that their traffic numbers are going up up up, but in my opinion any true value will be seen once the hype settles down and users become a bit more mature.