Facebook was built on the _____ of earlier social network sites like Myspace and Bebo.

A. succeed (v) succeed

B. successful (adj) successful

C. successfully (adv) successfully

D. success (n) success

The position to be filled is a noun because preceded by the article the

=> Facebook was built on the success of earlier social network sites like MySpace and Bebo.

=> Facebook built on the success of previous social networking sites like MySpace and Bebo.

The answer to choose is: EASY