Where do you get these "Facts" from that "most social networking sites are broke" ???
Any high-traffic, targeted online advertising can be VERY profitable. Think of it as TV ads for kids being put on kids shows during the hours they're home from school. What Facebook, Myspace etc. do is even more refined and targeted than that.
I think Facebook will be around for many years to come. They're at the point where other social networking sites have gone through ups and downs and they've learned from all of that, and they're continuing to add in new features and applications to keep it fresh (and fun I'm sure some would say).
I'd only sell enough as an employee to buy a new car, pay off the house, or get the cash to lock up in an education fund for the kids (if I had any!). The rest I'd let cook for a while longer