We have many dozens of venues in the Washington, DC, area, of which house concerts are just one part. Amazingly, this community seems to be able to support them all.
In addition to the house concerts I mentioned earlier, which are sponsored by the Folklore Society of Greater Washington (FSGW), there are at least three other regular house concert series (two of them sponsored by individuals). The FSGW concerts are a bit more traditional-music focused, while the others tend to book more singer-songwriters.
We are very lucky here to have the FSGW newsletter, which goes every month to a folk-music-appreciating audience of 3000+ people who are members of he Folklore Society. The newsletter lists any folk event (not just FSGW-sponsored) whose promoters get the info. in by the copy deadline. This helps build audiences for everyone.