Investors and analysts spent years clamoring for Facebook to start up an ad network, and earlier this year it obliged them.

And now Facebook is rolling out another one.

Caveat — this isn’t an ad network in a formal sense. But it’s going to be viewed as one, because in the big picture, it does the thing people outside Facebook wanted an ad network to do: It lets advertisers buy ads, via Facebook, on properties Facebook doesn’t own.

Some of the details about Facebook’s plans have been leaking out for months, starting with a report from The Information. Facebook is formally announcing its plans tonight, but is providing few details itself.

The broad strokes:

Facebook is reintroducing Atlas, the underused platform it bought from Microsoft last year.

Facebook says Atlas can help marketers track the effectiveness of their ads around the Web; it also says it will allow them to buy ads on non-Facebook websites and apps, using Facebook targeting data.

Facebook makes a point of saying these ads aren’t “Facebook ads.” But it is also playing up the notion that the ads marketers buy via Atlas will be more effective than other big ad platforms, because they use Facebook’s data.

Facebook says it is working with lots of partners, but so far has named only two. Ad holding giant Omnicom, which already has deals with Facebook, Google, Twitter and most other big digital players, says it will buy ads with Atlas. Facebook’s Instagram will also work with the platform. The most tantalizing notion I’ve heard this week is that Facebook has talked to Twitter about joining up, and that the idea remains a possibility.

What’s that? You’re worried about people using your Facebook data to serve you ads? Facebook says you shouldn’t worry, because your identity will remain anonymous to advertisers and publishers — they’ll just know some basic facts about you. But really, if you’re worried about this kind of thing you shouldn’t be on Facebook. Actually, the whole Web is probably a no-go zone for you. Sorry.

It’s worth nothing that this announcement is timed to the beginning of Advertising Week, a goat rodeo of New York keynotes, panels and parties, when many pointless announcements are made. This is not one of those. But we also won’t know what it really means for a while, because we’ll need to see what advertisers and publishers think of it.

That said, Facebook has been quite open about the fact that it is targeting Google’s DoubleClick display ad business with this move, which is interesting.

Facebook and Google are already neck-and-neck in the display ads business, but right now it’s an asymmetric battle: Display ads are core to Facebook, but they remain a bolt-on for Google, which still revolves around search ads. This year, for instance, Google may generate $4 billion in display ads in the U.S. — and generate $54 billion in net revenue overall.

But some smart people I talk to suggest that what’s really at play here is data, not dollars: If Facebook can convince more publishers to let it into their ad business, it’s ultimately going to glean information that will makes its own ads, on its own properties, much more powerful. Google will watch closely.