All database is not same

All database is not same.
In Object Oriented database, there is no need for link between two database. Facebook use hierarchy database. Example under a post there is comment. Under comment there is another comment. Under comment there is another post. It is object oriented database which Facebook use.

But

In relational database, link is needed between two database. In market, Relational database is not expanding.

So all database is not same.