LaunchRock adds the social validation thing. I'm not sold on their thinking there. (I'm using it to find my target market, which means I'd be ok if they were just able to track IF they shared, not how effective they were).
Unbounce has better reporting than launchrock, and some nice templates, but their social contest pages are smoke and mirrors. You can't determine if anyone shared w/ them either.
Both are free to try though. 50 signups on someone else's domain is simply useless, unless you're anticipating failure.