I'm not sure that the revenue they've begun to generate (which is reportedly sizable) is actually coming from that magical "social graph" rather than just plain old advertising (and quite spammy advertising at that), but that was the premise for huge valuations in the past.
So, facebook gets value from occasional users, and having occasional users go away does hurt them in the long run, since I guess the long bet with facebook is still the "social graph".
Regardless of that, negative growth in their most lucrative markets is a horrible sign for a company like facebook. Slow growth is one thing...but losing people by the millions? That indicates significant breakage. I don't know enough to know what kind of breakage, as I'm not really a facebook user (I login maybe once a month).
I'd say 10-15% of my friends list (age 24-28) have left Facebook.
Some users just focused on websites catering to specific communities.
Some just focused most of their time on Twitter/Tumblr.
And some just realized they got by day-to-day without a social network and elected to get out of the game altogether.
This is what I would guess. I suspect that one of Facebook's problems is that people naturally overshoot with their social network and at some point you end up with "too many" friends and logging into Facebook reveals less interesting content and more noise. I know this has certainly happened to me. 80% of the content I see upon logging in is information from people that I couldn't care less about. Facebook has encouraged this from the start by avoiding support for stratifying friends into separate groups. It's worked well for them by simplifying things, but I think they'll begin to experience some natural attrition from it as time goes by - social networks just naturally grow and grow because you very rarely "unfriend" someone. At some point it becomes burdensome to check it instead of fun and then it's all over.