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. Facebook is reintroducing Atlas, the under-used platform it bought from Microsoft last year, and it 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 Web sites and apps, using Facebook targeting data.