UD/Grid has more users than Folding@home, but most of them has stopped crunching for a long time ago, so when it comes to active users they're 2nd largest now.
As for the different BOINC-projects, a little difficult to say how many active... but users with non-zero credit is:
BOINC - 151.260 - TA - 279 thereof 126 active.
SETI@home - 101.633 - TA - 222 thereof 91 active.
CPDN - 38.418 - TA - 49 thereof 33 active.
Einstein@home - 37.432 - TA - 60 thereof 45 active.
Predictor@home - 15.707 - TA - 24 thereof 15 active.
LHC@home - 5.094 - TA - 11 thereof ... 3 active...
"active" is users got any credit in April...
CPDN has 70.793 registered users, thereof only 38.418 has got any credit, and has 31.850 active hosts last fortnight.
Anandtechs 49 active users has 188 computers, but it's possible someone have forgotten to merge duplicate computers...
Einstein@home has 73.403 registered users, 118.825 hosts thereof 61.257 has got credit.
Predictor@home had 6.788 computers return a result last 24 hours.
LHC is currently limited to 6.000 users.