The one-on-one aspect is huge. Our startup is trying to facilitate these kinds of informational coffee meetings:

Treatings: www.treatin.gs

We think there's something special about dating sites as compared to, say, Facebook or LinkedIn: everyone who has signed up has implicitly opted in to the possibility of being contacted by strangers to possibly meet up offline (though they aren't guaranteeing anything). Knowing that people have opted in like this makes most people more comfortable reaching out to strangers.