I 'think' the Folklore Society of Greater Washington is the largest...not withstanding those good folks in greater Washington state...*grin*
This may be partly because we have the Library of Congress folk resources here, many forms of ethnic music from the varied international communities, as well as having had several long-standing folk related radio programs to keep the 'folk' informed and entertained. (Mary Cliff for general folk, Dick Spottswood for, as he calls it "Obsolete Music, LOTS of Bluegrass...and various of the syndicated porgrams.

FSGW has a couple thousand members and many regular events, including a 2 day festival with 6 stages and a dance hall...about June 1, and a one day Mini-Festival in Feb....then there is another one day festival in Sept. in Takoma Park which we do not run, but which attracts most of the same people.

This has been going on since the 60s, which is why the now international Getaway ended up doing so well for over 40 years.

I occasionally have house concerts in my basement, and have had 40+ folks at several. (in the link above, you can see that Elizabeth LaPrelle will be in my basement in September)

Although there has been a decline in the 'critical mass' of hard-core 'folkies' in the last 10-15 years due to basic folk-entropy, you can find something of interest almost ANY night.

I can count 10 or more local folks who have a good repertoire of ballads.(including at least 3 who have sets of the full Bronson)

We do ok here.