I'll say this as a guy who is ridiculously bad at keeping in touch and keeping up with online people -- if you got a good vibe like you got on with each other then you can probably just shoot them a message again and say hello.
If you don't get a response either a) they're busy and taking ages to respond or b ) aren't interested.
If you got a vibe like you didn't have much in common beyond feederism, then it's possible they just weren't interested.
But the best way to go about it is to ask. I find that keeping up with people from here is easiest on an app like telegram or kik that has instant messaging.
I find if I have a lot in common with someone, I can talk night after night. If not, I usually just chat with someone once, and then run out of things to say.
