How to welcome your WordPress visitors WP Tip #009
If you install WP Greetbox, you can have a custom message to your visitors based on where they are coming from. You can change the look and feel of greeting messages by modifying the wp-content/plugins/wp-greet-box/css/wp-greet-box.css file too.
You can invite people that came from Twitter to follow you, or you can invite visitors from StumbleUpon to sign up for more updates that you'll send them from your email list.
There are other plugins that will help with this, for example WWSG which is short for What would Seth Godin do. The purpose is to greet your guests with a meaningful message. These plugins can detect whether the person came from Google, Twitter, Facebook or some of the other major networks.