They have more than 90% of the market. The fact that alternatives exist doesn't make them not a monopoly when hardly anyone is using those alternatives. See windows in the late '90s.
IIRC, Google tends to have more "real" searches. A lot of Bing and Yahoo searches probably have the query string "Facebook" because people have it as the default engine, or are already on a Yahoo / MS property (like their web mail). A lot of these people will then type "Google" into the Bing / Yahoo search box, when they need to find something.