There are several tools that measure different things:
Traffic (Google Analytics or some other system) -- here you may look at new vs. returning visitors, time spent on the site, number of pages viewed and learn if your social media site is sticky.
Influence as in reach (FeedBurner RSS -- important enough to be considered separately as they go beyond reach, they show the reader is vested in some way with the content).
Influence as in volume (all the social networks, Technorati, company articles submitted to social bookmarking like Digg, del.icio.us, etc.) -- you may know that the inbound links Technorati counts expire every six months. To be counter properly you also need to make sure your domain is mapped correctly.
Influence (Google Alerts) -- mentioned elsewhere but not linked
Twitter is a site/service where you send messages (max 140 characters). The messages can be submitted via the web, SMS, IM, or some third-part clients. People add you as a “friend” or “follow” you and receive your tweets as you send them.
Instantly notify customers of …
Quick messages informing staff of shift changes
Disseminating information / updates / product info