Is it ok to advertise on threads? I wouldn't think so, however, I saw someone advertising and just wanted to make sure of that.
There are designated places to advertise but sometimes people will spam. You can always help us locate them by using the "Report" button on the post in question.
Alejandro, you can always reference the Constructive Posting Policy which has examples but basically here are some tips that might help. Make sure that your comment furthers the subject. For instance, if a topic is about "3d cinemas" and your only comment is that you have not been to a 3d cinema then it really is not moving the topic forward. Otherwise, there would be so many topics that we can add to say "I have not tasted it", "I have not seen it", etc. I hope that helps. If you have more specific questions you can always ask in your own introduction thread.