It is widely accepted that Bitcoin is both a transaction network, and a currency subject to speculative market action like established fiat currencies. In a sense, it is a transaction network with a 'capacitor' in the middle, which happens to be stored value in the form of the world's first distributed digital currency.
I investigated and performed some analysis around Bitcoin Days Destroyed, and found it a little empty in terms of measuring the adoption of Bitcoin as a tranactional currency. Instead, I simply took a moving average of the number of transactions per block as a straight indicator of real transactional activity. This analysis makes no attempt to measure or attribute meaning to the size of transactions; the focus is exclusively on the volume of transactions.
To that end, the attached is a graph of transactions per block which includes a linear best-fit trend and a 255 sample rolling average. Both show clearly that the transaction count has trended down over the past few months. How valid is this graph as a proxy for the adoption of Bitcoin as a tranactional currency, and what level of activity should we want to see before we'd consider Bitcoin a 'success' as such a currency?
NOTE: in the graph below, the the slope co-efficient is transactions/(block*day), so the implication is that the average number of transactions per block is falling at a rate of 0.0716 per day, or about 1 transaction fewer every 2 weeks.