Branch, a Group Conversation Site, Publicly Branches Out - NYTimes.com

Another new Obvious communication/collaboration creation

"Branch entices people to start a longer, more in-depth conversation about a specific topic and invite others to join the discussion. Unlike most social networks, where anyone can join in and add their verbose opinions, only people involved in the conversation can invite others. This includes friends on Twitter and other social networks.

[...] 

 Branch, which has been in a private beta for several months, grew out of the Obvious Corporation, an idea incubator started by Evan Williams and Biz Stone, the two co-founders of Twitter, and Jason Goldman, a former Twitter executive."

Both Branch and Medium also use Twitter as an identity/authentication service.

Branch, a Group Conversation Site, Publicly Branches Out - NYTimes.com