I am a member of Facebook and have been able to locate a few long lost friends, including some are exJWs.
I have also linked up with some active and inactive JWs on facebook that I still keep in touch with. However, this means that other JWs are able to see who I associate with. This has got already one person in trouble. Do you think elders may start to use Facebook to track what the sheep are doing and who they are associating with?
I have noticed that some of the JWs have a lot more "worldy friends" on Facebook than JW friends. I personally think Facebook and similar sites are one more nail in the Coffin for the WTS. People stay JWs often out of fear of loneliness of being discarded to the world, and these sites help a person see how easy it is to meet people and become frineds of all walks of life.