IMHO they're not a monopoly, at least not in the traditional sense of the word.
Traditional monopolies "control supply of a good or service, and where the entry of new producers is prevented or highly restricted."
In this case, the supply of a good or service (social networking, webpage indexing and searching) are not controlled by Facebook and Google because they aren't finite resources. Anyone can index the web and anyone can build a social network and Google/Facebook aren't going to crush you with lawsuits or some other nefarious tactic to maintain their position (AFAIK).
Entry of new producers of these services is not prevented or highly restricted. New social networks and search engines pop up all the time.
What Facebook and Google have is massive, large scale user loyalty. Despite alternatives existing (Bing, Mastadon, DuckDuckGo, Myspace, etc.), users are voluntarily choosing to use Google and Facebook. This is not a monopoly.
It's used as a metric for the probably that someone will repay their debts; a metric used by those who are making financial decisions based on whether or not someone will do that.