This happened to me 3 times just last week. I reached out to all three through stored email addresses. One responded that he had recently moved and just wanted to take a break from it. Another, I found out through a friend, is now settled into a relationship and mostly just communicates through texting. The third has yet to respond.
Just like with its predecessors (Friendster, MySpace), Facebook is starting to plateau if not decline. Adults and teens are leaving the site for different reasons. Some adults cite the security issues, lack of transparency, the bad (and possibly criminal) IPO, and the fact that Facebook takes for granted its users and what they have to share.
Twitter is fast becoming the site of choice among America's teens preferring messenger apps just as their parent's are getting the hang of Facebook. Teens are leaving the site because they don't want to be on the same social site as their parents.
So there will always be something. The trick will be knowing what it is and how to use it before it loses its popularity.