Pass along being the key phrase. You don't use Twitter because it lets you post messages. You use it because the site is a giant infrastructure that is designed

Pass along being the key phrase. You don't use Twitter because it lets you post messages. You use it because the site is a giant infrastructure that is designed to pass the things you write along to other people, both through following and through replying. The actual system of posting messages is comparatively irrelevant: there were microblogging sites before Twitter, but they didn't focus on following other people, so they were forgotten.