Developers may make up to $500 million on Facebook’s developer platform this year, and now Facebook’s investors are getting into that action — and then some. The fbFund, a seed-stage investment fund run by Founders Fund and Accel Partners in partnership with Facebook, has announced the first 25 of its 50 finalists for this year.

These companies aren’t just on Facebook’s platform. Some use Facebook Connect to incorporate Facebook data on other web sites or on the iPhone, but aren’t on Facebook itself.

The finalists will get $1000 in Facebook advertising credit, and so will the other 25 finalists due to be announced later this week. A smaller group will be selected to win up to $100,000 and attend fbFund’s new incubator-style summer school starting in a couple months. Perhaps due to the growing revenue coming from companies on the platform — and Connect — the fund will begin making direct capital investments this year. This means Founders Fund and Accel will get equity stakes in the winners. Previously, fbFund only gave out grants.

Here’s the first batch of finalists:

– Connect sites: Frintro, GovIt, RunMyErrand, RentMineOnline, MyChurch.org, GreetBeatz, Workstir, NutshellMail, RunThere, DropPlay, Magellan (private beta), Life360 (private beta), Vittana (private beta)
– Platform apps: Travel Brain, Networked Blogs, Gameyola, Photos I Like, Paradise Paintball 3D, Veechi Classes, BitStrips (private beta), SamaSource
– iPhone apps: FriendFreak, Near+Now (Sortuv), Paparazzi, CrazyMenu

I haven’t had the chance to review them all. So here are a few descriptions that Facebook sent over:

– Frintro: Play matchmaker to help your single friends find friends of friends to date! Think social dating via “friendly introductions.”
– RentMineOnline: Combines the success of resident referral programs with the power of social networks. Residents refer their community to friends through Facebook to earn rewards and live with friends.
– Travel Brain & GeckoGo.com: Play the Travel Brain app on Facebook to choose your favorite travel spots; then search travel reviews on GeckoGo.com
– FriendFreak: Play a fun and social iPhone app to learn more about all your Facebook friends

For a mostly quantitative look at how winners from the fbFund’s past two classes have done, check out this article.