Currently twitter is too tech oriented for the mainstream audience to really gain mass appeal in the U.S. according to this article by Business Insider. The chart above illustrates referrals growth to the Gawker website network. Facebook and Stumble Upon are driving most of the referral traffic to these more mainstream sites.
Twitter definitely has the potential to go more mainstream as the service starts to mature. I’ve seen great improvements on making Twitter easier to understand. It is much easier to follow the “buzz” of a community with trending topics and “instant news”. You really can not do that on Facebook. I think Twitter has a lot of growth potential and will eventually move into the mainstream.