I like the simplicity of this. Here's your biggest problem I think: I just filled in a bunch of overlaps (which was kind of fun), but that's probably all i'm going to do right now & I'll likely forget all about overlap.me by tomorrow.
Somehow you need to get back in front of me--say next week--with some reason I'd want to return.
A big challenge is simply having enough users to give you the sense that when you're bored or lonely, you could rely on Overlap.me to help you find connection.
I figure "small wins" -- like the initial fun of joining your first overlaps -- are a step towards building this critical mass. They don't encapsulate a long-term value proposition, but they get the site part-way there.
Some of the early testers found the service quite addicting and were returning every day to see who had joined the overlaps they've created. So another opportunity might lie in building more of an experience around this sense of belonging/community.
Somehow you need to get back in front of me--say next week--with some reason I'd want to return.