[foulmouthboy]

How did they figure it out?

[pavel_lishin]

I imagine that perhaps they visited a site without logging in, or spoke with their friends who also frequent the site.

[petervandijck]

Yes, both of those. They also often have multiple (dozens of) accounts. Trolling seems to be one of those activities where you really get in the zone, because they seem to have infinite amounts of time and energy.