Jim,
Promote may be the wrong word. I should have said affiliated instead. They have only affiliated with public or "for profit" clubs becuase there was a large market to tap.
Here is some info I found that says where they are clubwise. Looks to me like maybe 5000-6000 clubs are not members. My bet is the number of members begin to decrease now and over the next 10 years.
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The five founding members began to invite other club representatives to their annual meeting and slowly added members to the USGA. Member dues started at $25 for clubs waiting to be declared full members. In 1939 all clubs paid $30 to maintain their membership. Club membership increased from 267 members in 1910 to 1,138 members in 1932, then fell to 816 clubs primarily because of the Great Depression and the start of World War II. By 1947 the USGA gained enough members to pass the 1932 membership level, and start a slow ascent. In 1980 the USGA roster exceeded 5,000 member clubs.
Aided by an unprecedented course building boom in the golf industry and a campaign to recruit new members, the USGA added 3,300 club members between 1987 and 1997, then added another 700 members by the end of the Millennium. Today the USGA club membership includes more than 9,700 golf clubs, golf courses and qualified training facilities.
In the late 1980s the USGA actively encouraged the formation of clubs without real estate. Today clubs without real estate have grown to a 5 percent representation of the total membership.
In 1995 USGA member by-laws were modified to allow enrollment of qualified golf practice centers and schools under the banner of "training facilities".
As recently as 1985 the USGA club membership was comprised of 75 percent private clubs. In the past 20 years the organization's profile changed to having 64 percent of its members organized as public or semi-private golf courses, public non-real-estate clubs and practice facilities. In the past five years, three out of every four new members had a public golf orientation.
Today, USGA member club representatives control or own 10,600 golf courses with 171,000 holes, providing employment for thousands of people and places of play for millions of golfers. More than 680 clubs hold qualifying rounds for USGA or state golf championships. More than 400 member clubs have hosted a USGA championship."