Robin asks an interesting question:

Bryan Caplan recently pointed out to a few of us that while many dating web sites offer to help you find matching romantic mates, there are far fewer friend finding helpers.  We tend to collect friends informally, by liking the people we meet for other reasons, and especially friends of friends. But for mating purposes we are more willing to choose folks based on a list of their interests, an intro paragraph, a picture, etc.  Why the difference?

His theory:

We need mates more for their simple surface features, while we need friends more to serve as social allies in our existing social network.  Since we need friends in substantial part to serve as allies in our social world, supporting us against opposing coalitions, it makes sense to draw our friends from our existing social world.  And since we need mates more for their personal quality, e.g., good genes, youth, wealth, smarts, mood, etc., it makes sense to pick them more via such features.

I have a different theory, though I’m not especially confident in it. First notice that people are actually often eager to make friends with people outside their social circle. They don’t want to make friends on the subway, but they join groups, play sports, couchsurf, partake in a huge range of social online activities, and go to conferences often with the intention of making new friends, who would be outside their existing social circle. A difference between any of these activities and online dating is that with the latter you have to be explicit about the fact that you are trying each other out when you go on a date. It is obvious when one of you decides against the other, and the relationship is usually sharply ended. With meeting friends casually this is not so; you can talk to people and assess them a lot before anybody even knows you are considering being friends with them. Even once you have done some friendly thing with a person, if you don’t see them for months its not clear whether you hated them or have just been busy. Friend meeting activities are best then with a group of people and a supposed other purpose to the interaction.

I think this latter style is necessary for friends but not for romance because you can have many friends and only one partner, which makes turning someone down as a friend much ruder. Turning down someone as a partner says ‘I don’t think you are the best mate I can find given a few decades’ whereas turning down someone as a friend says ‘you are worse than zero’. It’s hard enough to explicitly tell someone the first thing, the second is near impossible. And if the recipient doesn’t listen to the first, you can get angry, have them arrested, get your new partner to threaten them, or whatever. Would be friends can safely hang around for years not getting hints.

So if you were to advertise for friends you would probably be stuck with most of those you tried out, at least for a little while until you managed to gradually happen to not see each other, and there is some risk of the relationship remaining for a long time. These risks make online-dating style friend seeking just too costly.