Twitter is a social network site that allows you to post 140 characters message on your profile page. You can post message to general public or to a specific user. Message can be about where you are, what you are doing or even a link to websites. Unlike Facebook, there is no large profile page to tweak around.
Twitter had a headquarter office in San Francisco on 795 Folsom Street. On June 1, 2012, they moved to the Market Square building on 1355 Market Street. This blog post has more photos of the office.
Twitter headquarters.
Twitter accounts that pretend to be SF landmarks
Here's some notable twitter accounts that pretend to be SF landmarks or features:
