I'll try to throw some suggestions at you, see what sticks:

Online:
- meetup.com
- searching for a specific hashtag in Facebook and befriend the people that shows up
- Kickstarter

Offline
- fliers with your contact info and the intended interests
- a post on some wallboard in a crowded community (that is somehow related to the field of interest)

I've always been told something along the lines of "find a group based around a hobby that you like/ an interest that you have, and make friends through them," though I've been recently wondering if it's possible to guess in advance which groups might be more likely to contain the most potential close friends.

Personally, I've had an easier time making friends with bronies and HPMoR readers in meatspace than I have had with making friends with people participating in, say, service organizations or chemistry club. The most obvious explanation here is that I have more in common with people in the first two of these groups than I do with people in the last two of these groups. Still, I'm nevertheless tempted to posit something about the fact that signaling membership as an HPMoR reader, or as a brony, is reasonably costly to some people-- and that this might serve to filter out a portion of the would-be members of these groups who I'd be less likely to be friends with.

The most time-efficient way is to be open about who you are and what kind of people you are seeking so that other people can find you.

Find a community that someone else created of people with similar interests and skills (eg, lesswrong if you're looking to cooperate with other people interested in rationality