This is a very crowded space already. My advice for you is to find you a particular niche and see what appeals to that community. Design your social networking site around the targeted community.
You can start one for schools, or Old age homes.. they would love ur adsense blocks... You can look around facebook/bebo/ for the features I also have an idea for one... and the features, you can copy them when I have launched the stuff
i got a targeted community but now i'm doing is to find a features that may be interest with young professional.