The FARs are a massive document and are updated asynchronously. As a result, we can't always be 100% sure that we have the absolute latest. But, don't get too frazzled by this - remember, this also means that any FAR put to paper is nearly instantaneously out of date!
We update FastFAR somewhere likewise irregularly depending on the volume and nature of FAA changes. Use FastFAR only where you'd be equally happy using a FAR/AIM book published not too long ago.
Speaking generally, FastFAR is MORE up to date than any FAR/AIM bookout there. (though of course some books may be more up to date at the instant they are published, in general FastFAR will tend to be more current).