According to Wired, during times of peak usage, Netflix Instant streaming accounts for 20% of internet bandwidth used in North America. That's a staggering figure — more than YouTube or iTunes — though Netflix video is higher quality and therefore more bandwidth-heavy than YouTube. (At least if you're marathoning something relatively recent, like King of the Hill, Season 13 — Netflix tends to encode older, less interesting things, like Kubrick movies, at lower quality. Grumble, grumble.)
Will Netflix eventually kill the entire internet? This question gets more interesting when you realize, as Wired points out, that only two percent of Netflix subscribers even use Netflix Instant. That number has nowhere to go but up, as physical media inevitably become obsolete. We're going to need a bigger internet.