[triceratops]

You're asking what prevents straw purchases. Same thing that prevents straw purchases for alcohol and tobacco. Plus once it's the norm to not have kids on social networks there will be greater societal enforcement, just like with smoking and drinking.

There's no database of alcohol or tobacco buyers. The sales clerk just checks your id and forgets about it. It's a code good for use on exactly one account on one service, and expires in maybe a year. If you have multiple accounts on multiple services, buy separate codes for each one. They should be reasonably priced - no more than $5 each.

You don't need a perfect solution. Despite laws against underage drinking, smoking, and drug use it still happens.