I mean something like if there is a thread about let's say the lack of something that the service provide, is it allowed for the service to go there and tell "you know, we are doing it !" ?
I will say yes, but very very carefully. It is incredibly easy to interpret someone advertising their service in a related topic as spam. If someone made a thread and is asking for a specific service that fits their needs and you tell them about your's, that is fine. If you go into all related topics and start posting about your service because they are marginally related, you are likely to have the post deleted.
I suppose the best way that I can give you a helpful gauge as to whether it would be appropriate in a thread with your service info or not is make sure its not disruptive in any way. If your post is not completely on topic with the original topic of the thread, it is likely spam. If its an unwanted post in their thread, it is likely spam. An example of that, is if someone was complaining about being in a car accident, and you offer to sell them car insurance, sure the two topics are marginally related, but likely unwanted.
