This means that 22% is actively used in the bitcoin economy
Except that it doesn't mean that.
That's the thing about a pseudonymous digital currency -- the blockchain data reveals so little about its usage.
We know that Mt. Gox and other exchanges use cold storage. Thus the coins moving to and from the exchange's hot wallet represents a lot of monetary velocity, and the vast majority of the bitcoin value is held in static coins in cold storage.
Individuals might follow a similar pattern. A big chunk of their coins are stored in a secure manner (on a paper wallet, or air gap system) and left untouched, and a smaller amount used on a mobile or wallet used for daily spending from a standard, network-connected computer.
Because Bitcoin works not just as a payment network but as a currency, even the exchanges won't show the degree of commerce occurring. Online gambling through SatoshiDICE, for instance, is not cashed out to USDs, and players don't necessarily need to use an exchange to get bitcoins to play. So much of those flows likely never touch any exchanges (except for players buying bitcoins or cashing out if they were a big winner perhaps).
Other merchants though will still route bitcoin revenues through the exchanges. Coinabul, for instance, at a current revenue rate approaching a quarter-million USD worth of trading each month (based on numbers from an interview by Julia of @DGCMagazine), converts revenues to dollars right away after each sale, apparently.
As Bitcoin moves up the supply chain though (and/or is used for salaries, paying dividends, etc.) more and more of those bitcoin revenues will remain as bitcoins without landing at the exchanges.
So it is going to be really tough to measure the blockchain, or even monitor it for trends, to tell what really is occurring with the Bitcoin economy.