You want the answer? Start adding some of those people to the main project. If they're writing the proper tests and the code is solid, add them. If they botch a release, remove them and roll back.
There are two scenarios:
1) You aren't responding to pull requests in a timely fashion
2) Your original code isn't meeting the needs of the users
If there are that many active forks, consider inviting the most active contributers to IRC and finding out why they aren't making pull requests. If they are and you aren't responding then the blame lies soley with the master repo maintainer.
The answer is a proper, centralized project, with a mailing list for communication and coordination, and for the forkers to at least make a stab at getting involved.
If that's not working, one of the forkers should fork the project, not just the code.
Projects are things involving people, when all is said and done. Community and communication are what make an open source project work, not the ability to bounce code around with push/pull requests. Those things help, because they're more efficient than mailing patches around, but people have been doing this stuff for a while without github...
Good point about the mailing list. At this point though, it seems that just finding a way to deal with the multitude of forkers is the biggest issue. That might be a mailing list or it might be a shoutout on github asking for information.
I'm not too keen on a big structure off the bat when you don't know what the reason for the forking is. Then again with google groups it's really not that big of an issue to set one up.
And I'm fully aware that people have done this for the longest time without github but right now, github is where the "community" lives. It would be unwise to go shunting the community around to some new thing without first establishing what the community is.
There are two scenarios:
1) You aren't responding to pull requests in a timely fashion 2) Your original code isn't meeting the needs of the users
If there are that many active forks, consider inviting the most active contributers to IRC and finding out why they aren't making pull requests. If they are and you aren't responding then the blame lies soley with the master repo maintainer.