When there are seasonal variations, comparing this month to the month a year ago, or this quarter to the quarter a year ago is pretty common, not just in financial circles, but also in other circles. For example, the community college I work for compares Fall Term numbers to last year's Fall Term numbers to see how we are doing, and likewise Winter Term to Winter Term a year ago.
By comparing to a year ago, seasonal variations are automatically accounted for (e.g., summer vacation effect on bidding and funds available) without having to come up with a model that adjusts for seasonal variations. Besides, Prosper doesn't have a long enough history to be able to predict seasonal variations.
Looking at month-after-month numbers is also useful, but without a long history, one really doesn't know how much of month-after-month changes are due to seasonal variations.