The twenty-something CEO made the announcement to a group of developers in Silicon Valley, all of whom, it would seem began blogging immediately (TechMeme has no fewer than 70 links on the subject from press and bloggers – interesting TechCrunch’s sudden dominance at that site, with Arrington trumping the NYT, Forbes, BusinessWeek, Scoble, well, everybody; Gabe still crashing on your couch sometimes Mike?).

Sixty-five companies had about 80 Facebook applications ready to go upon this announcement, including Amazon’s book and CD recommendation widget, and integration with Microsoft’s Popfly, Virtual Earth, and Xbox Live.

The open platform allows deep integration into the Facebook community, mass distribution in the network, and great new opportunities for businesses to connect with a growing social network.

Buried under all that widgetizing and blog-buzz (the digilepsy) was also the release of Facebook Video, double the company’s assault by adding YouTube to MySpace to the list of competitors. The player isn’t embeddable elsewhere though; it looks like Zuckerberg wants to keep users on-site.

Since opening up beyond college students last fall, Facebook’s membership has doubled and is growing by about 3 percent (1 million) weekly, putting it on pace to catch up with MySpace relatively quickly.

For marketers, the announcement paves the way for highly targeted advertising. Robert Scoble explains:

It’s going to let Facebook deliver very targeted advertising…. Much more than many more popular sites. Look at Digg, for instance, Can you figure anything out about the users of Digg? No. So, how will you send targeted advertising to them?

Some interesting facts about Facebook:

6th most trafficked site in the US
Average 20 minutes per user per day
Over 40 billion page view monthly
Half of Facebook users are out of college