It also depends on the service. The cheapest way to advertise on Facebook is to buy one or more active facebook pages and post on them, of course, not to go to people's brains ... :) Of course, buying the pages itself costs money, not least for good sites, but in the long run, if you do it right, it can pay off in abundance. Free solutions are usually banner exchanges and the like, but they are not very effective unfortunately. Another paid option is Google adsense, which, if done well, can be brought out cheaply per click, but it is not effective for all services. For example, I also made the mistake of advertising my facebook page with a Facebook ad. There were also a few likes, but it was a waste of money. On the other hand, it worked for another site that deals with interesting videos. It also shows that the subject is very different.