Fascinating comment last week from Bitcoin insider Edmund Edgar, giving us background on my observation that 2016 looks like the year Bitcoin transactions peaked (see above). According to him, it's even worse than that:
So what's actually happened is that there's a cap in the protocol about how many transactions you can have per second (or to be technically precise, the size of the block containing those transactions), and we basically hit that cap at the end of 2015. Rather than just raising that cap the Bitcoin community had a huge factional religious parameter war, and it hasn't been raised in 2016, so the number of transactions hasn't changed.
Demand to send transactions still seems to be going up, so the cost of sending a transaction has been increasing. I think (although I'm not confident) the amount of value in each transaction has been tending to increase, so effectively low-value transactions are getting priced out, and it's being used for progressively higher ones.
I believe the transaction cap relates to blocking Bitcoin "gold farming" from behind China's Great Firewall. Anyway, argues Edgar, it's changed the underlying utility of Bitcoin: