What the Fallacy?!
Bandwagon
The Fallacy
Appealing to popularity or the fact that many people do something as an attempted form of validation.
Why it's wrong
The flaw in this argument is that the popularity of an idea has absolutely no bearing on its validity. If it did, then the Earth would have made itself flat for most of history to accommodate this popular belief.
Example
Everyone in the office was buying the new 'miracle' diet tea, so Mark decided it must be scientifically effective, even though the ingredients were just sugar and grass.