I get that making a topic/response only to share a referral link is considered spam, but then again, what qualifies as making a topic only to share a referral link?
Does writing a lengthy post about what a service is and how it works with a single referral link below the whole thing that considered spam too?
From my experience yes. IIRC the only time you are allowed to post a ref link is in a well rounded response. E.g. if someone asks for a good service for X and you reply with a well rounded review you may add a ref link if its clear that it is one.
I am not 100% sure on that though, but I think I read something along those lines. All in all, I would say if the ref link is worth it, put it in your signature and otherwise dont bother.