I am not a economist, but I am related to one...
While it's technically true to say that "Currency is only worth what people think it's worth" and that it's a socailly-constructed value, you are ignoring the underlying economic reasons why people assign greater value to one than the other.
The value of a US dollar is based on the power and stability of the US economy and Federal Government. No matter how bad things may seem right now for the US economy, it is much better than trusting a random internet craze, and no one doubts that it will be around in 100 years, hence people buy 100 year bonds. Even the currency of a small country like Sweden is a better bet than bitcoin.
I've looked into bitcoin, and while I think the idea is cryptographically sound, there is one problem with the concept: While there are built-in limits to inflation within bitcoin, there is nothing preventing someone else from building "Bitcoin 2" or "Crypto-coins" with the same concept but different keys. If merchants are willing to take bitcoin, what is to prevent them from also accepting any other crypto currency, thus devaluing the whole pool?
The value of currency as an investment is dependent on how much it will be worth in the long run, and while I am sure "bitcoins" will be around in 10 years, what will its value be? Will be around in 100 years?
And this is ignoring the issue this article bring up, that with a newly-consructed pool of currency with much fewer users, it is much more prone to currency manipulation than dollars or euros.