you mean storing everything on the blockchain?

it sounds very interesing.
i guess you can do that. everyone has a private key that they submit "stuff" to the blockchain and others (their friends) can see those "stuff" using their public key to find them. but everyone else would also be able to see it if they have your public key since blockchain will be public. so you won't have the privacy option of the facebook. (unless you implement the anon transactions in it).

the other issue would soon be that it require a very huge space to hold all the data and that can make it centralized again in a couple of data centers.