It's been asked many times for hotels and airlines here in different threads, and in general, I don't think there has ever been an answer. They all hold those numbers very tight.

United has said this

. We do publicly confirm the number of people in the (Mileage Plus) program — (more than 54 million) — but we don't typically break it down. It's interesting to people. (At) the top end of Global Services, (they may wonder) how big that is and "how many people am I competing against for my upgrade?" (Just) as it is for general members in terms of just wanting to understand the sort of scope and scale of the program.

In USA today a few years ago (April, 09)

A flyertalk thread from 2001 puts the delta number at 28 million, but that's way outdated probably.