A message no longer has a "shelf life", but a "wavelength".

For those struggling for ideas, you could build a "has this prize expired site, tracking these very RTs".

A lesson learnt which they failed to mention is increase the time of your competition to greater than 16days ( or whatever it was originally )

That said, for people who are not pushing competition and are using Twitter and RT's to generate leads or help users find content from them. It gives a slight indication that many many trickles might not be harmful to you. ( Well thats at least what I have come away with. )