If you understand blockchain well, you'll have an easy answer to that.
One answer I have in mind is, blockchain is seen by these social media giants as a powerful technology — powerful in a sense that it is decentralized. By being decentralized, it won't have to be controlled by any central authority or government. There will be reduced (to zero) taxes, easier transaction based on smart contracts, lesser manpower on the part of FB and TW (hence the huge money that can be saved).
Any visionary would see blockchain taking over most business transactions in the near future.
you have some good points but for the most part you are wrong.
first mistake is that you said "blockchain is decentralized". well it is not. it is just a technology and it can be used both centralized or decentralized. for example bitcoin is using it decentralized, XRP, ETH, the government coins like Petro of Venezuela, the coin of Indian gov., the Iranian coin,... are all centralized usage of blockchain.
second mistake is that you think these companies are using the "blockchain technology", they really aren't doing that. what they are doing is simply creating a centralized token for internal money transfers and they are doing that purely out of greed to make money because things are hyped up with this technology these days.
there will be no smart contracts, no decentralization, no innovation, ... there will only be a centralized database which might as well be like Paypal.