If they are, they are not a traditional monopolies. 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 n…
Most, if not all, websites will block your bot after a couple of request even if it respects robots.txt.
Also they seem to have different rules for GoogleBot and different for yours.