It's my feeling that the "stickiness" associated with these services lies in putting one's personal life on display. Certainly, users can maintain a certain amount of control, however there seem to be some "loopholes." In fact, the Boyd and Ellison article mentions that friends of friends can gain access to a person's account. I think this is referred to as the "transitive property" in math.
Some will argue that the connectivity networks like this brings, outweigh any possible negatives. However, I do know for a fact that employers search MySpace and Facebook sites to gain additional information on prospective candidates. This step is considered as routine as a reference check.
Use of these social network sites have become addictive and allow users to escape the realities of their regular lives and take on created personas. As seen in the FrontLine video segments, these personas and connections have led to users dealing with issues in a destructive way. Use of these sites allowed users to disconnect with the family and friends who would have helped and supported the user.
Despite this negativity, more and more people are spending increased time on these sites. I believe a big reason for this is peer pressure. You aren't someone unless you have an account on one of these sites, and often you will be left out since this has become the new mode of communication.
Another reason for the increased addiction to these social networking sites is the technology and many ways the user may customize their account with colors, skins, etc. Just as we can lose ourselves creating our avatars and trying on new outfits and hairdos, the user of these sites can customize and customize without end!
While I know conceptually, there are many positives to social networking sites, I am afraid my strong sense of privacy will always be a hindrance in openly accepting these sites.
