Personally, if I were to look for a co-founder for a project, I would most probably go only with a person I know personally. It's very unlikely that I would trust someone online for a project(especially for a co-founder position); though it completely depends on what 'project' you're planning on doing.
What project are you planning on making in the first place?
I am trying to create a coin and an online bank for crypto, People can take loans. I just started so I don't have all the technical specifications. Basically a blockchain based bank.
So.. pretty much like the Salt lending platform[1]? Anyway, if you're doing a project that's decently big in size, I'd probably socialize in blockchain meetups. I don't think looking for potential colleagues online isn't going to do much good if you're just starting out.
[1]
https://saltlending.com/