It is the size of course. If all of the Facebook subscribers were to open a facebook account and fund it with a substantial amount of money, this may just become a self-sustaining economy. And this is where we have to do some numbers. How big will a self-sustaining economy look like. I am sure that there are economists that will be able to guide us in this, but my gut tells me it is something like twenty million accounts (at least), with an average balance of $500 to $1000. Anything substantially less will not have critical mass.
Unless the value proposition of Facebook credits are much bigger than what it is currently, I cannot see that these numbers would ever be achieved. Should we take Facebook currency seriously? Depends what Facebook comes up with that will entice me to keep a few hundred dollars in my account... which I have not yet opened.