Facebook, Twitter, and Google are fair examples, but in each case, what makes these things hard is the server components. Now consider what it would take to bu

Facebook, Twitter, and Google are fair examples, but in each case, what makes these things hard is the server components. Now consider what it would take to build the next Final Cut Pro. You could try to do it as a web app, and you might even get some casual users who don't need much capabilities. But you are not going to displace existing professional tools on any platform. Being available everywhere just means you're not better anywhere. Java apps discovered this last decade.