Respondez s’il vous plait
Anyone who has ever planned a large event knows that the struggle of RSVPs is real. Respondez s’il vous plait is a French phrase essentially meaning 'please reply.' In most cases, your host is trying to ascertain a headcount for food, seating, and so forth. It is hard to plan events if one has no idea how many to expect.
Thus, if you receive an invitation - whether a formal card, an informal website, or a casual Facebook event - always acknowledge that you received the invitation. Then, be a good guest/friend and respond as soon as possible (or at minimum by the stated deadline).
Finally, once you have responded, you must stick to the response that you gave (unless you have a true medical or other emergency...and 'just not feeling it' is not a true emergency). Thus, if you've said you'll attend, show up. If you did not, don't come unexpectedly.
While some occasions - or friendships - may weather a poor RSVP, there are those that are less adaptable (plus, imagine if you weren't the only one who didn't follow that RSVP and your host ends up paying way more than necessary on food, only to speak to an empty room...or vice versa, and you get the last piece of chicken when those who did respond miss out!).