I think it has to do with each and every individual community, as each one has its own ettiquette and ideas of what is and what isn't acceptable. For example, I would probably consider it spamming if you went through the DaniWeb memberlist and started contacting them to advertise your website (even if it was computer related). On the other hand, there might be other [more informal] communities that have members who would welcome it. I really can't comment on your particular scenerio because I'm not a MySpace user so I don't know about their community.

The best advice I could give you is to browse around and get a feeling of the type of community atmosphere you're dealing with. And if you're still not sure, then don't. There's nothing more damaging to an online reputation / persona than coming off as a spammer. And, remember, usernames and website URLs follow you from community to community. I'm known as 'cscgal' on a good 50 or more tech forums, and I'm careful with what I write wherever I go because, in the end, it all affects DaniWeb's reputation.