Depending on how it's done, it can actually get more members than advertising on any site like this. For example, back when I owned my Free Domain Guide I joined forums without a domain and contacted the admins about using my guide to improve their communities. I made a few posts before hand, as to integrate myself with their site and learn a few details to "personalize" the message I sent the them. Out of 40-60 PM's I sent that way I earned around 25 refs and got 0 complaints
I personally wouldn't be annoyed if I got a PM advertising something, as long as it wasn't simply a mass message. I did get one of those at one point... didn't go well for the site owner.