Sadly that's the way RPG works. From the years I've been here, a few things I've noticed is that:
-About a 1/4-1/2 of those that post their interest in the interest check will vanish after their initial post or not make it into the IC/OOC. 
-A fourth of those that post a sheet either never post again or post a few IC posts before losing interest.
-Half of those that have been on the guild less than a week or two will disappear without a trace (although it's a bit hard now to tell who all is new since everyone from OldGuild had to start from scratch)
-The critical period for a roleplay is the first month or two in which most roleplays die. 
- If you can get a solid core group to last after the first two months your success level greatly improves even if new people trickle in and other leave. The biggest killers after the first two months is stagnation and chain-quitting.

The numbers are a little different in Advanced (and I'm guessing Free, although I don't have much experience in that section) and sometimes you can get lucky and prove these stats wrong if you get the right group of people.