Joel Gascoigne makes a good point:
The problem with social ideas is that often even though they may be solving a problem in a novel and useful way, they almost always have the following traits:
- take longer to validate that the problem is something people want
- product/service is not as useful when there are fewer users (network effect)
- revenue usually comes after some kind of "tipping point" which is hard to specify
Whilst I think social ideas can be great, my point here is that if you're starting out and want to be working on your idea full-time in the near future, you are much more likely to achieve that goal by building a tool which people find immediately useful.
Simply put, the problem with "social startups" is they tend to not make money. That's a pretty big flaw if you're trying to build a business (though of course some rare examples succeed despite that).