I have seen this claim a few times, but noone ever gives any real reasons as to why.

A monopoly born through a superior product is not the same as a monopoly born through anti-competitive practises. Noone is forcing or coercing anyone to use Google+. Facebook and twitter are only a click away, and both are massively more popular.

Could we say the same if Facebook launched an email service? Or if Twitter launched some sort of search engine?