Advertisers have long been used to paying a central service (whether Facebook, Twitter or even a newspaper) to push adverts into a reader's feed which they have no choice but to see. Facebook and Twitter (and Google) went further by adjusting algorithms so that users don't even see their friends' posts any longer in chronological order, but rather in the manipulated order the service provider determines.
This model is severely disrupted with decentralised, federated and peer-to-peer social networks that are rising up now, as they don't contain these algorithms, and neither do they push adverts into feeds. A user would have to follow an advertiser or business in order to see adverts. They would also see the adverts in a global feed, but with equal weight of any other post. So if say Ford had an account on a Mastodon instance, any user elsewhere on Mastodon, Pixelfed, Hubzilla, etc could follow that account if they wished to, and would see the adverts then in their feed. But it's a whole different mindset that advertisers and brands would have to adjust to.
Yes it gives users the power to decide what they want to see and will mean advertisers will have to work hard for relevancy. Even inserting adverts at the app level will be difficult as often there are multiple such apps available per network (it's a world of freedom of choice) and they are open source so anyone could just produce a newer app.
It's Facebook's extreme greed that has actually helped usher in this change, which is quite ironic. My wake-up call came last week when a very locked-in family member decided on her own to ditch her Facebook account - that really showed me the frustration that is taking hold.
See What a Decentralized Web Means for Digital Advertising
The prognosis for ad tech is not good