Such is the nature of disruptive technologies. The "yet" in your comment speaks volumes on the impending doom of traditional, multi-channel television. The core value of Twitter, however, is the fresh and organic nature of the information flowing through it. Netflix relies much more on quality than newness. The major difference here is that advertisers are paying to have information artificially inserted into the Twitter-verse where the core value is the organic nature of the content. A subscription greater than marginal ad revenue would greatly increase the amount of organic info on Twitter (both to subscribers and free users) which would lead to more users which would lead to more overall ad revenue.