If you really want to feel the need to follow your friends, no need for Twitter. Google has added the iPhone to the list of devices for its Latitude product. This is the “See where your friends are and what they are up to” service that allows you to track your contacts.
Instead of making this available in the AppStore, it;s a web-app that works through Safari. Google explained by saying “We worked closely with Apple to bring Latitude to the iPhone in a way Apple thought would be best for iPhone users. After we developed a Latitude application for the iPhone, Apple requested we release Latitude as a web application in order to avoid confusion with Maps on the iPhone, which uses Google to serve maps tiles.”
More here at Google