The United States religion census of 2010 recorded 41,436 members across 95 synagogues, and that remains the most defensible figure. A 2020 survey places Reconstructionist and other movements together at 4 per cent of America's roughly 7.5 million Jews, which is a combined category and must not be reported as a Reconstructionist figure. The movement claims something like a hundred affiliated communities today.
By any measure it is the smallest of the major American movements, and has been roughly flat for decades. Its influence has been very much larger, and the pattern is consistent: Reconstructionism proposes something, the larger movements reject it, and thirty years later they adopt it.
The bat mitzvah is the clearest case. Kaplan gave his daughter Judith the first one in 1922, and it is now universal outside the Orthodox world and common within parts of it. Patrilineal descent, recognising as Jewish the child of a Jewish father, was Reconstructionist before Reform adopted it in 1983. The full inclusion of women, and later of gay and lesbian Jews and rabbis, ran the same course. Lay-led worship of the kind now found across American Judaism came from the same current.
The structural question the movement lives with is why the ideas travel and the membership does not. One answer offered from inside is that a movement whose distinctive claim is that Judaism should be reconstructed by its own communities has, by succeeding, given the other movements everything they needed to reconstruct themselves, and left itself with no particular reason for anyone to move.