U.S. law does follow U.S. citizens around the world, but only in certain circumstances does it apply to conduct abroad. For example, there is a law against U.S. citizens engaging in underage sex tourism abroad. But such laws are the exception. If you, e.g., murder someone in England, the U.S. can't hold you accountable under American law.

Your example is a bad one, because in your example the law is broken by conduct in the U.S. In this case, the splicing of the leased lines happened outside of the U.S.

The conduct in this case is not the fiber tapping, but the possession and use of U.S. citizen data by the NSA. It does not matter how they gather it, their restrictions are the same--they must limit and justify it.

The Google+ comment objects to the fiber tapping. It doesn't say anything about the use of U.S. citizen data, and the NSA asserts it has safeguards in place to filter out such data from foreign taps.

It depends. Extradition is complicated and often has conditions, like requiring that the act was illegal both countries. Some countries will not extradite a citizen of the theirs (dual nationality comes into play here), and countries in Europe will not extradite to the usa if the death penalty is an option.