Since Bebo brought its Facebook Platform-compatible API out of closed beta a few weeks ago, as we reported in this post, the initial growth shows a steep curve from about 50 launch partner applications available before Jan 12th to 714 on Jan 26th. For a better sense of what that looks like, the graph below shows the developer uptake since public launch.
By way of comparison the total number of applications available on Facebook went from 13,060 to 14,509, an 11 percent increase. However, look at the absolute numbers. That's an increase of about 1449 Facebook apps compared to 664 Bebo apps.
While Facebook continues to grow faster, Bebo's growth is quite impressive growth for a new platform. Certainly their Facebook compatibility and the collective experience of existing Facebook developers is a big factor. Now the discussion becomes one of of general app fatigue, staying popular and monetization models.