How relative can be "Blockchain" and "Social Network Applications"? We hear too much of using Blockchain for "Social Network Applications" (such as Facebook or tweeter) !

I really do NOT understand how blockchain  can be useful for Facebook or tweeter ?! Where, data in social networks applications are not (and must not be) immutable and transparent to everyone.

Assuming someone in one of the social networks applications writes a new comment, this comment will then be verified in a central server of that company and if everything would be right, the comment will appear in the App. This user may then decide to delete this comment. Under some conditions, it will be disappeared from the App. 

From my point of view, implementing such scenario based on an open and permission-less Blockchain is not feasible at all. And if Blockchin is private and permissioned, then chaining blocks (Blockchain) do NOT make sense, where the entire chain can be replaced by another one by that company.

Am I wrong?

Can anyone explain one of the use cases of Blockchain for Social Networks?
I think you have pretty much answered the question yourself. The only use case I can think about for implementing social network on a blockchain is if you are hosting content that is illegal (connecting the likes of drug dealers, child porn makers etc.). In that case you can be assured that since there is no single point of failure, the law enforcement agencies cannot shut you down. Consider Silk Road Dapp on Ethereum. For all other use cases, it is redundant.

The current hype of putting everything on the blockchain is a gross neglect for the basic engineering design principles. Blockchain has a very specific use case: Ensuring transactions are valid and no double spend occurs. It should not be used to store your house land record, your marriage certificate or how many times you shower in a day. As you store more and more redundant data on the blockchain, it would get so bloated that a normal user would refrain from running a full node. What's the point of calling your blockchain as a decentralized system then?

As far as my interactions go with normal people, they do not mind seeing advertisements next to their social media feed. Some just block it off with ad-blockers, other love the recommended shopping lists it shows them. The mere concept of paying $0.1/tweet is absurd and pointless. You can build pretty elegant software solving the real life problems that are currently being faced by people. Putting everything on blockchain is definitely not what is going to solve them.