See this Rooms blog post and Facebook’s Rooms App Is a Flashback to Internet Bulletin Boards (NYT) for more details
"The app is like an infinitely large house where users can create rooms for specific purposes, like discussing current events, planning exotic vacations or breeding yorkipoos. The posts, which can include photos or videos, appear in chronological order.With New App, Facebook Harkens Back to Internet 1.0 - Digits - WSJ
Like Internet message boards, all posts in Rooms are public. But the creator of a room may limit who can contribute posts. Contributors can use pseudonyms.
Rooms is the latest entry in Facebook’s strategy of building standalone apps independent of the social network itself. This year, it launched the news aggregator Paper, a photo-sharing product called Slingshot and a video-capture app called Hyperlapse."