It sounds to me like Facebook never designed their software to scale efficiently (understandably so), but instead of overhauling their inadequate code when red flags started appearing (surely 10's of millions of accounts ago) and planning how to properly and efficiently run what would end up being one of the internet's busiest websites, they just kept throwing more hardware at it (not a terrible short-term strategy, but a stop-gap at best). Unfortunately, with their current size, they've probably now reached the tipping point and hardware upgrades alone can't continue to compensate for inefficient programming.
Facebook is probably hoping that, by going on record blaming underperforming hardware, they can have a scapegoat in place to blame for inevitably more frequent gripes about website performance as they continue to grow.
I can't remember ever seeing another major internet company publicly question the performance of the servers that host its website... as far as I know, AMD, Intel, Sun and the other chip and hardware manufacturers catering to the webserver industry have a pretty good performance track record.