Well, about a month ago my spouse firewall very nearly failed entirely. The problem was that with her mature Facebook account, with dozens of "friends" with a ton of other "friends", the stream of continuous bad news just became too much. Partially that's particular to some relationships we have, but even ignoring those, it was still an unrelenting stream of people having surgery, complications, financial troubles, and then very bizarrely mixed in with cute puppy photos, and then mixed in with political shitfits, to say nothing of some other family's "drama", all mixed with a "healthy" helping of continuous ads.
As it turns out, my firewall "recovered", but usage has been toned way down, and a lot of people got themselves hidden.
It gets said on HN every so often, but MySpace and the previous social networks often collapsed virtually overnight. I still think there's a good chance Facebook could still go that way.
Edit: Sorry, my "spouse firewall" is that my wife is on Facebook and I have no account. I hear about the family gatherings and other such things announced only on Facebook that way, but don't have to have an account myself. So it's like a "firewall" filtering out only the most important stuff.