The interesting thing about online self-promotion is that it can be vital to your success, but can also make you look horrible, tarnish your reputation, and alienate people. There are good ways to promote yourself, and there are bad ways. Some of the bad ways are often looked upon as spam - things like blog comment spam, forum spam, or of course Twitter/Facebook spam come to mind.
As iFroggy's Patrick O'Keefe said in an interview we did with him at SXSW, it's just disrespectful to a person's space or community. "All kinds of online communities have their own form of spam," and people always cross the line, he says.
O'Keefe also offered a couple of good tips for doing self-promotion right:
1. Look for the best people or most popular people - the people in a community that are already doing it right. Who's using the forum right? Who's using Twitter right? Watch and learn. Look at their practices.
2. Look at the guidelines. Online communities will often have terms of service, FAQs or some kind of posted policies. Read them.
If a blogger or forum expresses a policy against links in comments, then don't insert your link. Chances are that if you do, you'll look bad to the rest of the community, and that's not the kind of promotion you want.
Unfortunately, not all communities have readily available policies, but you can still observe how the rest of the community interacts with one another, and most of them aren't just throwing links around, you should probably acknowledge this and take it as a hint.
I realize that this is common sense to many, but at the same time, many, many people just don't seem to get it. Either that or they don't care. But they should care, because if they really want to earn business, this is just not a good way to go about it. They may not realize this kind of behavior is looked upon as spam, but if you're one of these people, I can assure you it is most certainly looked upon this way.