However, this may also be a function of age -- I'm in my early thirties, as are my friends. And of course, New York City.
I do think you're right about "online dating works well for people who are already likely to be successful at dating" -- in my experience, whether it's online or offline is irrelevant. Online dating isn't any fundamentally different at all -- people are attracted to each other, or not. Online dating just vastly widens the pool of potential people you might meet. You might see 100 people in a bar one night, and 90% are immediately out of your age/gender range, leaving just 10. You could easily browse through 200 filtered candidates on a dating site in an evening.