B. And unlike a neighborhood, where one usually has a general knowledge of others who live in the area, social networking sites are gatherings of [cut-off] individuals, none of whose personal boastings and musings are necessarily trustworthy.
The best replacement for the word deracinated in the sentence is cut-off.
Explanation:
The definition of deracinate is to tear something up by the roots, the sentence presents the example of a regular neighborhood where people are close to each other and there's a feeling of belonging, people have roots in there, on the other hand, social networking is not land for anyone, people don't have roots and are just "deracinated" or "cut-off" kind of people.
