Quoting a rule from the unofficial list of official Bitcointalk rules:
in others threads'
Based on my experience, linking to a product means advertisement and the post will get removed, but talking about it means that it's fine. Well, that's a false thought in my judgment.
My interpretation of this rule is that you shouldn't promote for instance Stake in Primedice's thread, but it's okay to mention them elsewhere. I'd say "thread hijacking" shouldn't be allowed.
I sometimes mention or even recommend a certain exchanger for a certain task. You could consider that advertising, but in my opinion an occasional recommendation (when appropriate) is different than someone who recommends a specific service in all their posts.