First off, Know that EVERY successful idea implemented by any techie faced massive criticism, From Steve Jobs to bill gates to mark zuckerburg, they strived and they succeeded.
The mass market appeal was occupied by a competitor, or was simply not there but people like them made it. Bill gates had a passion for computers in a time where the future of computers looked so bleak and dim, people thought it would be an immediate failure, now look! During the rise of Facebook there were countless other social networks, but Facebook presented a unique concept and people adhered to the calling of the ' next big thing '.
if you went back and told bill gates ( who had no funding, his father was a wood chopper ) that computers were going to make him a billionaire, no doubt that he would laugh his arse off.
Do you have the will? the desire? the passion? GO FOR IT, and don't let anyone tell you otherwise.
Also, it is very naive of some people here to consider Facebook so popular, it is only popular HERE. Japan, china, Germany, Russia all have their own versions of Facebook that you probably have not heard of, and they have a massive following.