How do they know ??

I created my Twitter account and within a couple of hours or so I picked up four "followers".

Each of these was basically a semi-spam account. Real businesses (as far as I know) all related to poker - with something to sell. This happens a little bit on facebook. In my case, for example, a cardroom or casino may invite me to be their friend. I usually decline such meta-advertising.

On LinkedIn, headhunters invite you to connect so they can get in touch with you and so that they can see into your network of colleagues.

Fair enough, I suppose, and I nearly always decline those invites as well.

On Twitter, I doubt one can block a follower - it's a "recipient led network".

There's no real harm in it either.

What what intrigues me is how - when my account was just tens of minutes old and I have made no posts at all - they knew I was a poker player ?

They'd be kind of scary at the table I guess - excellent reads.