Zynga is losing millions of its daily active users, according to data on AllFacebook.com, dug up by John Sweeney of the Facebook & Zynga Blog.

If you go back one month, CityVille had 21.1 million users. Today it's down to 19.7 million.

Farmville has lost over a million users in a month, going from 13.8 million to 12.7 million:

Here's Mafia Wars, which had a sharp spike at 5.5 million users thanks to a promotion before easing off to 2 million:

What's going on here? Zynga's games come on strong, and then fade out just like any video game.

It's another reminder that Zynga is a hits driven business and it needs a new hit to keep its strong momentum going.