Unity also doesn't have the network effects that Twitter has. People use Twitter because the people they follow use Twitter, and those people use Twitter because enough other people use Twitter that it's the easiest place to build an audience. Network effects do exist for game engines (it's a lot easier to use a tool when enough other people use it that the solution to any given problem is likely just a web-search away) but the critical mass that needs to be overcome to become competitive is a lot lower.