FB and all big companies are a very bad example. They have ton of resources and usually they don't use vanilla products, since they have the engineering capacity to support their own forked versions. e.g. see their own version of PHP.
Also distributing reads is easy, writes... not so much. NoSQL systems usually offer distributed writes with the caveat of eventual consistency. RDBMS have referential integrity and other constraints which by definition cannot migrate into a distributed environment. Or at least there's not a one size fits all solution.
> Distribution comes into picture from shape and size of the data not data saving/retrieval techniques. yea?
Most definitely not. It has nothing to do with the shape and size of data. Also.. there's not such thing as "distribution" in our context. Only "distributed", from "distributed computing"[1] and it's everything to do about data saving and retrieval :)