or
http://coinmarketcap.com/in my opinion it is very hard to measure it precisely. And I would say, perhaps it cannot be done exactly on last penny or cent.
People tend to think that when they name and describe world around them they will understand it better they will uncover the mystery...
Sure it is usefull to measure market backing of BTC but for different people it would have different meaning.
For example for me, when I dont have many bitcoins, it is only waste of time, to be frank.
I am intersted in cryptocurrencies, I love this ideas and of course, I hope that my investment will grow in value
(I think it is also (at least theoretically for sure) possible that general wealth of cryptocoins enviroment would grow and rise almost for everybody but "nobody hurts" - on the contrary of forex where earnings of winners are from loss of the unsuccesfull 99,9 % rest).
So basically I am intersted in core ideas and I intend to hold coins for long time. And what I could do with precise market backing of BTC? to be frank, I dont care if it is 0.1 or 1 or 2 or 3 billions. I care about my little investment and use my common sense.
I am also sure, if it could be measured somehow, than it would make people tempt to influence somehow this number since they could earn some quick money on it.
Because I think maybe only 10% of bitcoin are traded and the rest is stashed in computers. so BTC Market Capitalization of $ 1.26 bln could only mean 10% of the sum or even less. And some btc whale with pile of bitcoins could easily move the threshold of $ 1.26 bln according their needs up and down.
However I watch different opportunities and (currently) I prefer to hold other coins because they grow much more.