That doesn't mean it has to be that way in the future though. There are centralized social networks that are charging money for their use, and they are not selling to advertisers nor are there any.
With decentralized, you still have the same issues (I don't really know how you solve those unless data transfer is truly peer-to-peer which I think causes discoverability and similar issues to usability), but at least the scope of them is inherently limited and you can opt in/out of the source based on your trust of it.
It doesn't matter how it works, the fact of the matter is that while logistically third parties are typical in today's environment, they aren't implied by centralization.
Does Amazon's s3 sell your data to third parties? No. It's still centralized though!
> Since you're not going to add a government (or an ad company) to your friend list, they won't have your data.
Except when people use things like hashbase.io because they can't afford to run the server program 24/7 themselves. Then the hashbase.io analogous service sells your data to a third party, and congratulations you've got a system that has all of the lose of distributed systems, with all of the lose of centralized systems.
How is that going to work then? Every single company holding your data in a central place is mining it today and selling the information to advertisers.
On a distributed network you replicate data only with people you choose. Since you're not going to add a government (or an ad company) to your friend list, they won't have your data. Private communications are encrypted and flowing directly to the recipient, so there's no port that agencies can tap into to listen. There won't be any big data to analyze, since data is spread across the network. Backup is easy, it's just a folder. Fake news never gets promoted, since there are no paid promotions.
Spam is a non-issue since there are no advertisers. There's no way to get into your feed than through your friends. There's no server or central storage, so employee misuse is not an issue. And you can have as many identities as you choose, there won't be a mandatory real name policy.
I could go on.