I don't generally use PM systems on websites. It basically boils down to this: if it's something that I don't mind other people seeing I feel that I have an obligation to let people see it (as it may be useful to them), whereas if it's something I do mind other people seeing I shouldn't be putting it online in the first place. Especially so on a website like Hubski, where people choose the content they wish to see. Worse comes to worse, you move it to a new post under a different tag.
If I had any money, I'd have a domain and give contacts unique email addresses under said domain. Doesn't keep them from figuring out who I am, but at least stops the spam issue.if it's something I do mind other people seeing I shouldn't be putting it online in the first place.