From Jan'13 to now, which is when most of this merchant adoption has occurred, I can see an increase from around 40,000 transactions to about 60,000 transactions, which is rather modest and one can easily cherry pick points which show that the number of transactions has actually declined.
ed: And like I said in my other comment to vijay, it's not even clear that much of this represents positive-sum economic activity.
"40,000 transactions to about 60,000 transactions"
Exactly. You called this +50% growth "more or less constant", which is why I corrected you.
As to judging how much of this +50% growth is due to increased economic activity, it is impossible to prove due to the anonymous nature of Bitcoin. However indicators like the growth of Bitcoin payment processors (BitPay and Coinbase) very clearly show there is extreme growth. Eg. look at this data about BitPay: https://news.ycombinator.com/item?id=7974197
I do think that 50% growth in a year, especially when it is not that clear-cut that it really is 50%, isn't that impressive by itself. Perhaps there's a lot more growth in off-chain activity.
I remember reading an estimate that Bitcoin has < 500,000 users which seemed quite small to me given the hype surrounding it. So I'd be interested in seeing if there is growth in the number of wallets holding bitcoin. If bitcoin is taking off, we should be seeing that number explode, right?
ed: And like I said in my other comment to vijay, it's not even clear that much of this represents positive-sum economic activity.