The internet in Dec 1991 was already the largest computer network in the world according to some author back then. Certainly it was very very big.
Let me describe what ads or ad-like things there were.
Companies would announce job openings. These announcements were restricted to a few newsgroups such as ba.jobs and could not announce independent contractor openings: while it was okay to announce an opening for an employee (W2) or for an individual to announce his availability for employment, it was against the rules for a company to announce an opening for an independent contractor (1099) or for an independent contractor to announce his availability for work -- that was considered too much like commercial activity and was kept off the internet, which had almost no rules in the early 1990s, but one clear rule, consistently enforced, was, no commercial activity.
The job announcements stayed nicely contained (namely, restricted to newsgroups whose names ended in ".jobs") such that a person wouldn't encounter them unless they went looking for them. What did not stay nicely contained were announcements of academic conferences. When I was reading comp.lang.lisp, I could not avoid encountering announcements for many academic conferences on various programming-related topics even if they weren't about Lisp. And these announcements were repeated often (weekly or even more frequently).
But as far as anything ad-like, job announcements and conference announcements are all I can remember, and again only the conference announcements were obtrusive.
I heard that the proprietary online services like Compuserve and Prodigy had successfully argued in Washington that it was unfair for private enterprises to have to compete with a government-subsidized service (namely, the internet) and extracted a commitment from Washington that any commercial activity would be kept off the internet.
Again, there were almost no rules on the internet of 1991 and earlier. Tim Berners Lee for example did not need to get anyone's permission to start the web: he just wrote a web client, stood up the first web server and announced their availability. People acted like complete assholes on Usenet, and there was no way to rein them in. Pedophiles openly exchanged practical advice on how to target and exploit children on alt.sex.pedophilia. But one clear rule was no commercial activity, e.g., no selling, no trading and certainly no advertising.