Who are Twitter's actual competitors?

[Kevin B]

Facebook is for sure #1 at the moment with the new updates with hashtags and live news, acquisitions and competition in the market. But are there any more majorly significant competitors? I've read that Tumblr is a competitor but i just don't really see it. Both are labeled as "microblogging" sites but to me they are so much different. Tumblr is way more visual (although Twitter is trying to inject more of this) and is not up to par in real-time news as Twitter is. Google, still trying with their social media service could be one, but is there really any current threatening competition?

Of course there are many low-profile competitors to Twitter, but besides Facebook, which are the major ones?