I don't know if this is what jnbiche was referring to, but maybe the transaction volume isn't growing particularly fast. I don't know how useful/descriptive these numbers are, but from a quick look at blockchain.info's charts: the absolute number of transactions per day looks like it's only up by about 20% over the last year [1]. However, the USD value of transactions conducted is up by something like 200% [2]. I don't know how we could tell what fraction of these represent actual purchases, as opposed to individuals just moving their own coins around...
Actually the number of transactions grew +90% year-over-year, which is pretty fast growth! At +90% year-over-year, Bitcoin will reach 600k transfers/day in 3.5 years (what Western Union does today). Compare 2013Q2 with 2014Q2:
[1]: https://blockchain.info/charts/n-transactions?timespan=1year
[2]: https://blockchain.info/charts/estimated-transaction-volume-...