I'm not going to say this is the start of the end, but it certainly shows that people are no longer "excited" about the social network world.
People are noticing real-world consequences to the privacy issues, reducing the amount of interesting stuff that people will post. I've run into employer issues, I've had relatives with relationship issues, and I've read enough about legal issues to be wary about what I post. Add in the number of businesses embracing Facebook, and you start to see why "social networks" have reached a saturation point.
It still has its uses. It is more personal than email (great for keeping your family updated on life events). It is easier to control reoccurring events, such as birthday parties and pick-up games in the park. It is also easier to ignore people on Facebook than through email. I will continue to use it about as regularly as I use my email, but that doesn't mean I like it.