What Does "Stickiest" Refer to in Regard to Social Media

What does "stickiest" refer to in regard to social media?

A) Users who "stick" other friends to the site
B) Social media sites that use cookies to "stick" users to their site
C) Social media sites that connect users with other social media sites
D) Companies that use social media to "stick" separate companies together in order to gain customers
E) Users spending a great deal of time at a particular site