I think we're looking at this from an adult perspective. To say "just stay offline" to a kid, when they know everyone else IS online isn't so easy. At that age most kids want to belong, and it's torture to think something is going on that you aren't in on. Also, online bullying is an extension of what happens offline - it isn't as if they're talking shit about the kid on FB, but cool with them when they see them in person. That kid is getting fucked with around the clock. It's also different for boys and girls. Boys might be more likely to deal with it straight up, but girls are more passive/aggressive - they bully by starting gossip campaigns and doing sneaky shit behind each other's backs, and the victim of it is more likely to withdraw.
"The best kind of pride is that which compels a man to do his best when no one is watching."