As far as I remember it started as that. But maybe that's not accurate. I just remember them pushing how twitter could be monitored and then important tweets could be send around the organization to deal with them.
That feature is a flow (or channel) specific "inbox". You can add different sources such as e-mail, GitHub pushes, CI results, or the Twitter you mentioned. Basically looks like an in-channel RSS feed of different sources. Personally I love the feature.
A neat thing is that you can start a discussion thread from any of these items, so the context of the discussion is clear.
Oh I like FlowDock a lot was just trying to explain how I believe Slack won over it by being less opinionated about how it approaches conversation. It's IRC core functionality and then apps on top for specific needs.
Flowdock went a little further ahead when it came to how they constructed the base communication.
