It's quite simple Facebook is banned in certain countries and english isn't a native language for many of these countries. Considering this, the other local social networks have more specific features suited for the local crowd such weibo and qzone for china. Korea also has one. Weibo has more features than twitter. The principle of "less is more" doesn't work well with many of these other countries and their social networks.
That's discounting first-mover network effects before Facebook became as international as it is today (VKontakte in Russia) and the big case where Facebook is outright banned (China). For the rest, you have a point.