Multiple replacements for each company would appear within 24 hours. One would spread virally and gain an initial base of users, then press, and within a few months it would be in a dominant market position filling the void left by the American company it replaced.

This is exactly what happened in China when Google left - Baidu exploded in popularity and now dominates the market so completely that it would be nearly impossible for Google to unseat it.

The Chinese government does this on purpose to facilitate the growth of its own domestic companies. The Great Firewall serves as much of an economic purpose as a political one. By degrading service quality of foreign companies, it opens space in the market for domestic companies to establish themselves and gain market share. It would be impossible for any one of those companies to bootstrap its initial userbase if a giant American corporation already cornered its market.