Update on April 24, 2020:

The Facebook Creative Labs app Rooms shut down in December 2015. For information on Messenger Rooms, see this post.

Original post on December 17, 2014:

Over the past few months, people have created tens of thousands of rooms using the Rooms app for iOS to connect over common interests. Starting today, we’re introducing a new way to discover and join many of these communities via the Explore feature. Read more at the Rooms blog.