I realize that attending meetings, conferences, groups, etc. is a great way to meet developers in real life because they're already out there to begin with, thus, they have an interest in social engagements. But, the Internet has been proven to be a great way to meet like-minded people, e.g. romantic partners, as evidenced by dating sites like Okcupid and networking sites like Facebook. I've had difficulty finding people with developer forums, and I'm curious now to see what ideas people might have...
The best answer is to use the Internet to find meetings, conferences, groups, meetups, etc in your area and then attend them.
The next best answer is to get involved with a common project (open source is great for this) and then eventually people will want to meet you because they already know about this.
But there isn't a pool of developers out there who are thinking, "I'm going to a developer dating site to meet other developers." The ones who want to meet other developers are already going to meetings, conferences, groups, meetups, etc and meeting all of the developers that they want. So even if you found a site that purported to serve that need, there wouldn't be anyone there and it would be useless.
That said, it is easy to overlook useful groups unless you know about them.
One approach is to look for a particular technology, then look for user groups in your area that deal with that technology. For instance I know of user groups in the Los Angeles area for Perl programmers, sysadmins, Wordpress users, and so on. Once you find a user group, attend their meetup, see a talk, meet some people. An active user group generally meets once a month. If a user group is not active in your area, it is not nearly as hard to get one going as you would think. Some people have started a lot. For instance http://www.pm.org/ was founded by http://blogs.perl.org/users/brian_d_foy/ because he travels a lot, and wanted to be able to meet with Perl people when he went to a strange city.
There are also events that tend to move around that cross across skillsets. For instance http://startupweekend.org/ organizes events in different cities where a group of people comes together, splits up into small teams, and each team tries to launch a startup in a weekend. http://en.wikipedia.org/wiki/BarCamp is another worthwhile thing to look at.
Or for a more regular event, look for http://hackerspaces.org/wiki/Hackerspaces in your area and attend.
Computer Science Canada would be an example of a site that has some programmers and Computer Science students coming together on-line that may be an example of what you want.
Course you may also want to consider what kind of intentions do you have in wanting to find other developers as another aspect to this question:
Depending on the intention, I could see different possibilities of what to consider as different strategies may make sense but also there may be fewer developers wanting to go down a particular route,e.g. if you wanted to offer a course in teaching Commodore 64 BASIC that may not get a great reception today or so I'd think.
Sites like Facebook and LinkedIn have groups that you just may not know is another possibility to consider here. Twitter would be a bit more public and more of a long shot in terms of asking such a question.
Getting out and talking to people face to face would be my suggestion though there are on-line sites that may help you know where and when to go to find groups of developers.
A site to consider is Meetup: http://www.meetup.com/
Go there and see if there are any existing professional groups that meet near you.