I sent my inviations out last week. It clearly said at the bottom 'RSVP by xxth April to Milna's House, Milna's Street, Milnaland'.
So far I've had one written RSVP (passed to me at work by a colleague), one RSVP by email, and two RSVP's by text! I guess this is going to be the norm ... should have just invited everyone by text ! ?
I always send a card to RSVP if I receive a wedding invitation - I don't really mind the texts and emails, but it makes it a bit harder to keep track of - I have to remember to update my list (on the computer at home) every day before they get lost/I forget - written ones I could just pile up by the PC and tick off the list when its convenient.