If network effect means quality or “price” (in terms of what you trade, whether or not its money, for the experience) changes don’t lead to substitution between the dominant site and other sites, it absolutely is a monopoly, even if it would be equally easy to go to other sites. Monopoly is an empiricial, behavioral question of substitution between notional competitors, but not abstract question of available alternatives.

I'd say not monopoly, but anticompetitive behaviour like leveraging company's size, reach and cash to "forcefully" acquire competitors.