the mistake is to think that others have failed because they were not good enough. If 10 people tried before and failed, you can be darn sure that there's a good reason for those failures, and it's not lack of ability. When it comes to online software products, businesses always fail due to 2 problems: #1 no one wanted the product (or not enough) or #2 cost of acquiring customers was too high. These aren't going to be solved with better code.
The product has to be good enough to solve the problems of the market. #2 is also a downside of this - you can't dump marketing money to get around PMF.
Facebook is an interesting company which had a poor market (people are fine without it) but they managed to improve the product, the economics, the scale, just enough, and ended up one of the biggest companies in the world.
But, we're in established markets now. People have been using FB and many other sites for many years now and those behaviors are very much ingrained in people. There are only so many hours in a day someone can be online, so to some degree all new applications are competing with FB and the other incumbent apps. It's much much harder today than 10 or 15 years ago.
Established markets still need PMF. And existing sites/apps can lose their PMF.
There's a large demand for, say, a more privacy based social networking website. Or a gaming based social networking site, like what FB-Zynga used to be.
Google has a large blind spot too. Yelp is filling one, Pinterest covers their image search weakness, tons of travel apps/sites are filling another. But there's many, many things that can be more localized or made into a niche.
I'm not sure if it is harder, as technology has significantly improved. I think we're at the stage where less of the effort is building/designing an app, and more of it is in better product development.
