Is it? It has few 100s of millions of users, most of them in US - it's insignificant anywhere else. Older people don't know what it is, and younger people don't care about it - there's not enough dancing videos on it I guess.

Almost all people in politics use it. It's mostly 1 way traffic, but yes, politicians outside US use Twitter a lot.

Outside the US? Maybe in addition to other media. Definitely not as their primary channel. They would be completely unknown if they did so.