What were the others? Google Maps, MapBox, OSM are the three that come to my mind.

Waze? But my hyperbole aside ("only one"), how many people do you know use these other platforms? Colloquially people call things monopolies if they have sufficient market share, not absolute (which nearly never exists), or market collusion exists (e.g. ISPs, airlines, oil). People even do it for 2 dominating companies (e.g. Coke + Pepsi only controls 71% of market share (46.3+24.7)). Because the truth is that monopolies aren't always bad, they are just dangerous because they have so much weight that they can perform abusive tactics and that's the thing we actually care about. My whole point is that this gets to be a very sticky situation when the product is the market share (the more people that use Google Maps the better Google Maps gets. But maybe social networks are a clearer example).