To expand on what Manishearth said and address the last part of the question: the guidelines on what constitutes excessive self-promotion are outlined in the help center.
Avoid overt self-promotion.
The community tends to vote down overt self-promotion and flag it as spam. Post good, relevant answers, and if some (but not all) happen to be about your product or website, that’s okay. However, you must disclose your affiliation in your answers.
If a large percentage of your posts include a mention of your product or website, you're probably here for the wrong reasons. Our advertising rates are quite reasonable; contact our ad sales team for details. We also offer free community promotion ads for open source projects and non-profit organizations.
Suspensions for promotional content are issued when people behave in a way that violates these guidelines, are warned about it*, and don't stop. In particular, you need to make a lot of posts (questions, answers, comments, or chat messages) with only marginally relevant (or irrelevant) links to get suspended for that reason. No matter how many links you put in your profile, they only appear in one place, and it's a place that people actually have to actively seek out in order to see what you put there. So I'm fairly certain that nobody will ever get suspended for posting promotional content solely due to something they put in their profile.
Note that none of this depends on which site is being linked to, except to the extent that the target of the links determines whether they are relevant. (Linking to a porn site will probably never be relevant; linking to e.g. Quora could be, but it depends on the target and context of the link.) In particular, we don't have any rules of the form "no linking to [specific website]," whether in profiles or elsewhere.
*I should clarify that we do not have any policy saying that you must have been warned about certain behavior prior to being suspended for that behavior. But as a courtesy, we (moderators) usually try to verify that a user who behaves in a way that warrants a suspension is aware that they are doing so, and has a chance to stop, before actually issuing a suspension. There's usually not much point in suspending someone who doesn't realize they're breaking a rule.