Don't forget compression too. And don't forget most tweets don't use close to the full available tweet data space. A billion tweets takes up a lot less than 140GB.

I'm just looking at rough comparisons. A single 1/24th of a second of Netflix video approximates 44,000 tweets' worth of data. Anticipating growth, let's assume a billion tweets per day (was half-billion last October). 171 days of tweets takes up the same space as the complete Netflix library.