[skinnymuch]

Some admittedly silly examples of monopolies that fit the definition you’re saying. I know some one on HN before posted a much better example. I can’t recall it now.

Foursquare’s Swarm has a monopoly on social network checking in to places with optional photos + logged notes.

Tiago Forte has dominance in courses/in depth, varied content on building a second brain.

Readwise.io has dominance in spaced repetition learning from synced data.

There’s only one or two sites/communities for my personality disorder that doesn’t get much attention. They dominate the market.

There’s only one app that has you put up money for habits/challenges you complete via video selfies. Spar App (side note: horrible ownership/stewards of the app)

There was previously one community forum for video game site owners.

[almost] all of these have [near] 100% of their market. I don’t believe people would consider any of them monopolies.