If it's a real need, then building it is a good idea, and will probably make you money. But 99.9% of the time, it's just a turn of phrase way to bitch about the way the world and human kind actually works. Yeah, if we had another Facebook the world would be better. And if I had wheels I'd be a cart. What's your point?
We don't have another Facebook, and we aren't likely to get one, because we don't actually "need" that enough to pay tens or hundreds of people to build it. You just want to wish it into existence.
TL;DR: shut up and build it, or stop complaining.