What will user reaction be to this social-networking site's overhaul? Twitter revealed a redesigned site at a press conference Tuesday, hoping to make the site more user-friendly and more open to advertisers. The new site, which will roll out over the next few weeks, makes it easier for users to merge everything one wants to see on one screen–videos, photos, tweets, and conversations. Twitter CEO Evan Williams said the redesigned site will “increase the value people are getting out of Twitter,” which has about 145 million users. But the new site could threaten other start-up companies such as TweetDeck, Brizzly and Seesmic that produce applications specifically designed to make Twitter easier to use. The new site will also be more ad-friendly, as it will not force users to click away from the site to see photos and videos.