Just as the music industry has struggled with giving music away and the phenomenon of downloading, so too has the stationery industry struggled a bit with the issue of ecards.
As we've seen with music, I don't think ecards are going anywhere. And, in truth, I think ecards vs. real cards are totally different markets. Though there are certainly instances where sending an ecard will appropriately take the place of sending a real card, for many other instances, a dancing puppy just won't do.
Having said that, I love ecards. And in a way I think they get people thinking more about stationery and all of the reasons we should be using it. I'm even willing to pay, if the ecards are snazzy enough.
The NYT has an interesting article on the whole ecard phenomenon.