If you combine everything that you do on every website besides Netflix and YouTube — reading this article, or looking at GIFs, or checking your email — all of that combined generates about the same amount of peak web traffic as those two sites alone. Netflix itself is bigger in every metric than HBO.
According to (estimated) data compiled by Sandvine, a manufacturer of broadband technology, the video-streaming sites comprise 32.3 and 17.1 percent of all peak-period download traffic in North America, as reported by Variety. Combined, that's 49.4 percent. And at the rate that each is growing — 35 percent year-over-year for Netflix and about 24 percent for YouTube — it's a safe bet that the two will soon account for more than half of all the bandwidth we use.
Part of that is because video is bandwidth intensive, of course. Streaming video pushes a lot of data in a short time period, particularly at higher definitions. But a large part of it is because use of streaming sites is also increasing. From Variety:
Netflix said it streamed more than 4 billion hours of video globally in the first quarter of 2013, compared with 1 billion per month last June. The company has packed on customers, adding about 2 million U.S. streaming subs to stand at 29.17 million domestically — making it bigger than HBO in that regard.
Netflix has about 1 million more subscribers than HBO. But it almost certainly is also watched more, as it doubles down on binge-watching while HBO steps back from a streaming future beyond its cable subscribers.