Zucker

A person who uses Facebook too often

Steve, who has hidden browser tabs open to Facebook all of the time during work, is a real Zucker.

Factoid: Zucker, as a noun, can be modified in many ways:
"motherZucker": the mother of all Zuckers;

"Zuckerdoodle": a dog with its own Facebook page;

"Zuckface": a person who uploads umpteen selfies per day and/or changes their Facebook profile photo every day/hour/minute/second.