Sure you can clone systems, yet, there is only one facebook.
Facebook has real value though. You can't just clone facebook tomorrow, it would take millions of dollars of development time to recreate all the features of facebook. To create an exact clone of Bitcoin would take an afternoon. In fact there are already thousands of Bitcoin clones that are functionally identical.
So that leaves the fact that more people use Bitcoin. But that's no different than saying that Bitcoin is purely speculative. It's valuable because a bunch of people decided to buy it. Tomorrow those people may change their minds and decide that LiteCoin is where it's at, and there is no underlying value to Bitcoin. Network effect in Bitcoin (it's exchangable in a lot of places, and has sufficient volume to make it liquid) is far less powerful than network effects in Facebook which has incredible volumes of data on users and advertisers which represents a large moat for any competitors.