I am a J2EE developer who has worked only in apps that "take input data - store in db - retrieve the same and display" types of application. To move to social media kind of development job, what skills/technologies should I develop ?
In the latest IEEE Computer Society "Computer" magazine they have a good series of articles on social-multimedia.
You may want to look at that and then work on developing a framework, or extending one, to see if that is interesting enough to you, and to develop skills in this area.
If you try to work for technology that is a few years old you will be out-dated by the time you start, as I expect these companies are looking at the future and how to better incorporate multimedia into their social network frameworks, and, through their APIs they allow people to share across social networks.
A mashup that ties together several social networks, then adds something new, may be a good start, btw.
I'd suggest looking into the APIs of Facebook, Twitter, etc. that make up whatever social media you want to develop. While this is kind of like diving in head first, it may be interesting to see where this goes for you as once you start then you may find what area you want to use for a focus. There are also forums and other ways to get at some of the information but I would suggest a getting your hands dirty approach myself.