There were a lot of small announcements at f8 but this was the big one:

The ads come in three formats: standard IAB banners, standard IAB interstitials, and native ad units. Facebook says the ads are “designed to help marketers meet key business objectives, like driving app installs and engagement.”

...

Facebook notes that by integrating the Facebook SDK or working with a measurement partner, advertisers can track engagements and conversions driven by FAN ads, plus demographic info and other stats about their audience. Publishers can filter what categories of ads appear in their apps, so they can reject health, gambling or financial services ads but allow ones for food, education, or gaming companies.

...

Facebook first tested it mobile ad network in 2012, but paused it soon after to focus on its own native ads. At the time, the mobile ad network didn’t earn much money for Facebook because it merely a targeting layer that sat on top of ther ad networks that also took a cut.

In September 2013 Facebook rebooted the tests, but instead of a layer it had evolved to be a full fledged ad network that works directly with advertisers and publishers. This lets Facebook earn a higher margin. COO Sheryl Sandberg said the ad network tests “show a lot of promise and we’ve gotten good feedback from marketers” on Facebook’s recent Q1 2014 earnings call.

Re/code’s Mike Isaac reported that Facebook would officially launch the ad network today, and I confirmed the news with additional details including the name Facebook Audience Network and the fact that it would be compatible with both standard banner ads and custom native units. Now we have the first official look at Facebook’s mobile ad network.

This will be good for mobile app makers with engagement who want to run targeted ads.

And don't mind Facebook as a partner.

So the big players are now Google (AdMob), Facebook, Apple (iAds), Twitter (soon!), and inMobi.

Twitter MoPub is more a mediation service so Facebook Audience Network should work well with it:

The product will compete with other ad networks like InMobi. However, FAN will not compete with mediation services Twitter's MoPub. Instead, it can tap into it to simultaneously show ads from FAN and other ad networks. You can read more about how FAN and MoPub play nice in our follow-up article. While it may take a few quarters for FAN to spin up, it could become a serious source of revenue for Facebook.

The launch represents a big shift in how Facebook’s business works, from monetizing engagment on its own properties to earning money from its ad targeting data elsewhere. Until now, Facebook’s revenue has essentially been directly proportional to how many ads it showed in the News Feed and how many users visited.

The ad network will let it grow revenue without cluttering its own site and apps with more ads. That’s a more sustainable model that insulates Facebook from competition or a slow down of its own user engagement. If users are lucky, Facebook might earn so much from its Audience Network ads shown elsewhere that it could show fewer on its own apps and sites.

This IS a big shift for Facebook, and might ultimately lead to fewer ads on Facebook properties.