I highly doubt there is a "lack of competition" in China. For each and every one of those "Facebook, Messenger, Instagram, Twitter, Snapchat, Youtube, Gmail, Dropbox, Vimeo, SoundCloud" you mentioned, there are a hundred similar companies.

In fact, I'd argue that competition is rife and more ruthless in China than in Silicon Valley. The moment you have a good idea, ten companies will appear the next day hot on your heels. Even if the American stalwarts were to enter the Chinese market, who's to say they won't be crushed by Chinese competition?