...But what, exactly, is OP supposed to do about it? What leverage do they have in the world to change the way things are?
Realistically, when you’re building a startup, you use the tools you can get. Perhaps OP did not sufficient hedge across multiple engagement platforms but based on my experience, it’s very plausible that they tried multiple platforms and only found engagement on one.
The action they were taking was even to better understand their underlying demographic, presumably to approach they demographic from other marketing vectors because platform effect sucks.
Should OP simply ignore the opportunity because they haven’t been able to engage on another platform?
The problem with a monopoly is precisely that the monopoly’s users don’t have a choice.