I have some dating sites. There's a LOT of competition. Like most adult sites, you need to choose an under-developed niche/sector and exploit it yourself. Consider also setting up a dating site for just people in your area (state, county, province). You'll have competition but not as much and, because you cover a specified area, you can actually compete with larger sites that don't. For instance, say you live in New York. Your surfer is more likely to join an NY dating site than a worldwide site.
Be prepared not to make a cent though for 18 months
