From Wikipedia: according to the writings of Emily Post anyone receiving an invitation with an R.S.V.P on it is obliged to reply.
Notice that word, obliged. If the invitation is a mass Facebook invite to every single person the host has ever met in their whole life, feel free to ignore it. However, if the invite is a party invitation from a co-worker, at least send an e-mail stating whether or not you will be attending. If the invite if for a work related event, make sure to respond verbally and in the form the invitation came.
You’ll be thankful you followed common RSVP courtesies the next time you plan an event and need an accurate head count.