I bet most of the transactions are from gambling
The second coming from goods/digital goods transaction
And the third come from exchange site
I agree that gambling sites probably do make up a large amount of the blockchain transactions. At one point satoshi dice activity made up a significant percentage of transactions (although it has declined somewhat as of recently).
I would say that exchange activity probably would be second in my mind as I would think people would be constantly depositing and withdrawing when they are day trading, trying to perform arbitrage, or just buying to hodl. Plus when you deposit bitcoin on an exchange, the funds will almost always immediately move internally