In short: Although Bitcoin may have some admirable features as a medium of exchange, why do you suppose that it can represent a popularly desirable store of value when there are so many other excellent alternatives?
Note that a successful currency requires both features. Absent a reason to store value in bitcoins, people will keep their stock of wealth in other assets and then exchange them into bitcoins only when they need to engage in a transaction. But in that case, the real value of a bitcoin will be approximately zero; it will be slightly positive if the market deems its virtues as a medium of exchange to exceed those of other currencies, but beyond that, it will be zero.
Unfortunately, your understanding of the figures is a little off.
Growth in Gross Domestic Product (GDP) was negative in 2007:Q4, slightly positive in 2008:Q1 and 2008:Q2, and again negative in 2008:Q3.
Growth in Gross Domestic Income (GDI), which is logically identical to GDP but is measured differently and so can come up with slightly different figures, was negative in 2007:Q4 and 2008:Q1, slightly positive in 2008:Q2 and negative again in 2008:Q3.
Even more telling, payroll (i.e. employment) figures peaked in December 2007.
You can see all of this in the NBER business cycle dating committee's report here .
It's probably reasonable to assume that the small and temporary blip up back into positive territory in the middle of 2008 was a result of the first fiscal stimulus. It's also reasonable to assume that whatever caused it, that temporary respite saved the NBER and the Republican Party from an enormous headache. If the 2008:Q2 figures had been even slightly negative, there would have been significant (and reasonable) pressure for them to declare the recession in the middle of the campaign.
Moneyliness is next to Godliness. -- Andries van Dam