Maybe not impressive but none of the services of my customers had or has 1.8 million users. And yet they do well (my customers.)

That. And none of the big social media platforms were big at the start either.

Yes but 1.8Mi at a time where people are longing for a Twitter alternative is just leaving money on the table

There are two usual strategies for growing: 1) low cost, organic and slow or 2) high cost, throw a lot of money at advertisement, saturate all media, grow quickly or bust.

The exceptions are those rare products that despite a low cost marketing sell themselves so well that their organic growth is fast and in a few months everybody use them.

Maybe Bluesky don't have the money to advertise or is not compelling enough. As one data point: I know about Mastodon but I think that I learned about Bluesky only today. I went to their site and there is nothing to explain how it works except that it's some social thing. I learned more by reading the comments here. Apparently it's being marketed at a very low cost.