Meebo was many things but mostly it’s claim to fame is the Meebo Bar. The toolbar that allows users to share content on social sites and save it. It also provides for advertising and has a reasonably large install base. It is assumed that Google will be using this install base to drive Google Plus.
Facebook is highly valued exactly because of social targeted advertising, it is believed, widely, that socially targeted advertising is the next big thing. It is believed that people will want to buy what their friends are buying and everyone is trying to get a slice of the action, or to dominate and own the action.