The SDF did not publish membership figures during its first year of existence. To some extent this was an inevitable product of delayed formal entry into the organization by the various state organizations. At the February 1927 "Eastern State Conference of Social Democratic Organizations" James Oneal rather optimistically estimated the membership of the organization at "between 6,000 and 8,000" -- including in this count both the Finnish and Yiddish language socialist federations (the Finns ultimately did not join) and other sympathizers around the country in unorganized states (Wisconsin similarly may have been counted nor did they join).

The actual dues paying membership of the organization was undoubtedly far fewer.