Twitter is a niche in my humble opinion. They are basically a short messaging pubsub service for humans, and even artificially limited themselves to that niche, by not increasing message size limits in any obvious way, by not implementing other "social" features people actually use like better photo sharing, galleries and gallery sharing, games, etc. They didn't try to become a full-fledged social network and thus competitor to Facebook (and what Google+ was supposed to be) with messaging and a ton of other stuff.
Even that One Big New Thing they announced, namely Moments, still operated exclusively in that short messaging pubsub niche entirely (and turned out not to be something their users cared about that much so far).
Now their niche market seems kind of saturated, user numbers mostly peaked, and Facebook also started competing even more in that niche when they introduced searchable #hashtags and ways to follow accounts without becoming mutual friends, while still offering their other core features and "external" services like instagram (in direct competition to twitter) or whatsapp (in direct competition to twitter DMs). Tumblr is still there, which to me always felt mostly like a "twitter" for longer-form messages. I saw people switch from twitter to Slack (and recently Discord) and embrace their not-globally-visible pubsub models in some cases, so there is some competition there too.
So in conclusion: twitter seems to me like a pretty niche service (albeit large niche), and has competitors left and right now. It isn't in "that" niche, but in a niche.