Companies such as http://www.whitelabeldating.com/ provide a white label platform that allows you to launch a dating site with a shared user-base so that from day one your site has active members to search.
Even with an established user base, it's incredibly difficult to build a successful dating site due to the amount of competition. It requires a catchy niche and some seriously heavy PR & marketing to make any serious money.
Caveat: I work for the company that owns whitelabeldating.com
If you're thinking actual social space startup, that's a HARD row to hoe right now, unless you have a really crystal-clear new idea. It's a mature, saturated market that isn't in terrible need of disruption that I can see. The only really interesting entry in that field that I've heard of lately is Bang With Friends.
Plenty of Fish recently set their site so you cannot search for an "intimate encounter" unless you pay for Premium, as a result a lot of its userbase moved on.
Some links here: https://news.ycombinator.com/item?id=2126209