Yes, the parents of the girl whose birthday invitation went viral will likely regret inviting everyone. They probably already do, given all the hoopla about it. People are going to have to get smarter about how they use social media. It is an excellent forum in some instances but there are still times when it is not the most appropriate tool. Perhaps a birthday invitation is one such instance.
Inviting everyone to a birthday party often leads to unintended consequences. When that invitation goes viral only the chance for unintended consequences only increases. Sounds like fun, but the bigger the crowd gets the bigger the number of unsavory characters becomes. With unsavory characters comes all kinds of uncertainty and too often leads to violence.
Imagine having a bunch of strangers show up to your birthday party. Sure, you may run out of food and drinks for everyone, and you may be left with quite a mess. But having all those people show up would be amazing, and I imagine many of them would bring gifts for the girl.
I believe that if you are the type of parent who creates a viral birthday invitation, you know exactly what you are letting yourself in for. More private and reserved individuals would not even think to do something of this nature. These parents will not regret anything their viral invitation causes.