After years of rumors and tests, Facebook on Thursday launched its video advertising inside the News Feed, the newest type of ad unit for the social network to sell to marketers. Like many Internet video spots, ads will be 15 seconds long. They will automatically play inside of a user’s stream, though without the sound on (much like Instagram videos auto-play on mute right now, too). If you tap the ad, it’ll expand to fill the screen and the sound will kick in. Pretty straightforward. It’s been a long time coming. Facebook has been wary of upsetting its user base with the new ad unit, and has been methodical in its tests leading up to the launch. (So methodical, in fact, that the launch has been delayed multiple times.) And Facebook launched regular auto-playing videos last fall to get a good idea of how we’d respond to that type of thing showing up in the News Feed.