What makes services like MySpace or Facebook "sticky"?

What makes services like MySpace or Facebook "sticky"?
people spend a lot of time on these sites...why?

MySpace and Facebook (and even blogs can sometimes be considered part of this) are almost like the NEW homepage that we saw in the mid-90s. They are a persons digital, public face. People spends endless hours tweaking, adding content, adjusting etc. because this is what image they project to others. They want to look their digital best. I once was weary of a new employee because they showed no signs of a "digital-footprint" as I liked to call it. I was wary because having no "digital-footprint" is almost akin to not having a real-life footprint in this digital day and age. They were applying for a job in the IT department and this lack of virtual face, told me a lot about the potential employee. Mainly that they were not up on current technologies. I later proved to be correct in my assessment, but this just goes to say that having a "digital-footprint" or some kind of digital face is very important for both your personal and professional life. People spend lots of time on their Facebook, MySpace and Linked in profiles as it is like a "calling-card" its a way to enter into networks and be part of a community.