I started to use Facebook because most of the people I know have an account. It is the most accepted social platform available. The likelihood of contacting friends is higher than the competitors. It is also the easiest one to use that has enough support to stay afloat.
The addiction, I suspect, stems from two major sources:
1. The ease with which you can post a message your friends can see. We feel, once we have made a post, that our friends have been sent a personal message. In fact, this feels just as personal when we do it, but is really less personal. This lack of direct contact confuses our social requirements and we lose touch with what others want from us.
2. And then we have the games. The games give an illusion that you are participating in a large social game (like a board game on a grand scale). What they really do is give immediate rewards for clicking on things and the more often you return, the more often you are rewarded. This sensation of instant gratification, mixed with the illusion that you are interacting with friends makes these games feel important. Partly because it helps us feel like we are accomplishing something. Partly because it is easy to do.
We are creatures of habit. When we find something we like to use, even if it is inferior to other similar products, we don't like to change.
The end result to all of this is that we use what we know works for us. Much like Windows, there are alternatives, and many people will argue superiority. But until we can get the people we want sharing in the experience, it is hard to justify the change.