In the specific case of "Facebook for X", it's because there's a company that exists to do exactly that, it's called Facebook. More generally if your business

In the specific case of "Facebook for X", it's because there's a company that exists to do exactly that, it's called Facebook. More generally if your business model is "X for Y", where X is an existing company, you've possibly a lazy and uninspired USP which can easily be copied and your only real selling point is having the first-movers advantage and hopefully enough capitalisation that you can lead the market before the competition. You're just copying a business model and applying it to a different market, it's often a solution in search for a problem. I could make an Uber for farriers (the people who put horseshoes on horses), but is there actually a need? Do horse owners currently have a problem finding farriers? And if they did, some other company could easily make a competing app, and if they had more capital and resources could easily crush Horse-Shuber.