A(n) is an inducement that is awarded to social media users for passing communications along through different tiers of communities in social media.

a Viral hook is an inducement that is awarded to social media users for passing communications along through different tiers of communities in social media. 

To put it simply, Viral hook is a marketing program which pass one message to another in order to obtain attention from social media.
One of the examples of a viral hook that happened in recent years is the ALS bucket challenge.