I'm not sure why people keep saying this. I'm not reinventing either of these things but I suspect, like the services they each replaced, they too will be replaced with things that look strikingly similar to themselves.

For an oversimplified example, FB replaced myspace with a service that looked a lot like myspace did originally. One of the biggest things they had going was the simplicity of use and the cookie-cutter look of everyone's profile. Myspace, at the time, was full of 'pimped' profiles that made their experience suck.

Facebook will continue to add more and more fluff (games, etc..) until it doesn't work well as a tool to stay connected with people. Once this happens someone will show up with something that looks a lot like facebook did when it started. But because they're so dissimilar at the time, people will continue this adage of, "what ever replaces X won't look much like X". But it probably just will if you correct for time.