It already is. As soon as something like this is internet facing, you get search companies and now AI companies mining data to use for commercial applications.
In terms of the sites themselves though, it'll vary and depend. As it grows in populatity, there will be monetised content in plain sight (think all those secretly sponsored and advertising posts on reddits used to try and push products subtly - the bigger the user base, the more attractive it is to target users with hidden advertising), and then there will be what the servers do themselves. Some may exist on donations, but others may chose to try to place adverts, others may go for subscriptions.
Ultimately there does need to be money coming in from somewhere to keep the services going. There are many free success stories: Wikipedia continues to be free, without adverts, thanks to donations from users and sponsor organisations. Mozilla continues to produce a free open source browser through a mix of donations, sponsor organisations, and paid search deals. Linux is a huge free open system, with a mix of donations, sponsor organisations and commercialisation of the ecosystem.
There isn't really a reason why social media can't also be "free" for consumers, but we don't know yet how that will play out. On traditional social media, the user is the product - our data is mined, we're marketed at, we're advertised at, our data is sold on. The fediverse breaks alot of these methods - or more accurately it opens up these methods to everyone as anyone can access much of the data, removing the value companies have in monopolising and gate keeping the data. It's a double edged sword, but be in no doubt even in the fediverse companies can and will monetise whatever data they can get their hands on.