We do not presently have an answer. However, there are a number of possible sources for estimates.
First, several definitional problems:
- Do you include trade unions? (Arguably, the line between the two can be blurry.)
- Double-counting. If one person is a member of two organizations, is this 1 or 2?
I believe Bob Putnam -- author of the bestselling Bowling Alone -- compiled some of these statistics.
You may also be able to go to the source using one of the household/individual surveys that asks individuals about
membership. These are weighted so you can estimate from a sample of 1000+ households the number of people in the U.S.
with memberships.
The General Social Survey and Putnams own Community Benchmark Survey are two that are likely to be fruitful, although
I do not know if they distinguish professional from other types of membership.
Another possible source for an estimate is the National Association of Association Executives.
-- THP, 1/2008