Monopolies don't need to have all the market share, there just needs to be no viable substitutes. And a social network that is tiny with barely any content, is not a viable substitute. Another characteristic is high barriers to entry, which for social networks, includes overcoming the network effect as well as dealing with Cloudflare, Mastercard, etc. It doesn't matter if there are work arounds, if the cost of these workarounds is high, it's a monopoly.