I could make teddit.com, a reddit clone by copy & pasting their entire git repository at https://github.com/reddit/reddit. But I doubt anyone would use it, because of network effects. The same thing applies to bitcoin.
Your point is undermined by the fact that reddit hasn't lived up to its hype (and I believe its estimated valuation has gone down lately). Myspace was supposed to be worth hundreds of millions of dollars a few years ago. A large current userbase does not a stable value make.
