[pjc50]

No, there is not an implicit social contract in becoming a public figure, especially for those people who become public figures without their intent or consent (e.g "Star Wars Kid").

Also, if this was limited to criticizing ideas, it might be more reasonable. It isn't. For example, Caroline Crialdo-Perez getting rape threats on twitter for saying it would be a good idea to have a woman on a banknote.

To turn your phrase around, there is an implicit social contract in having a platform for disseminating ideas that you not use it for threats, intimidation, abuse or causing emotional distress.