Social networks tend to come and go. Yes, FB has been one of the "long lasting" ones, but nevertheless, they all have a tendency to come and go. At one point, nobody thought Myspace would go...and now it's a bit of a joke.
That being said,... I believe stocks will fall one day, along with FB.
To contradict my own statement above, though, I admit i'm not 100% sure. FB has implemented itself in most websites, allowing users to comment on articles using their FB ID. Sites like RottenTomatoes and others in the big leagues use this method. This is a very smart strategy that might keep FB alive for years if not decades to come.