Guessing: Meebo has been trying to make money via their behavioral targeting dataset. Meebo sees browsing activity of users via sites that embed their javascript and also through the plugins they get people to install in their browsers. It's a similar dataset to clearspring, who also do bt stuff. In 2011, Meebo bought mindset media who build psychometrics (eg [1]), with the idea of marrying psychometric targeting with meebo's bt dataset. Thus meebo's value to google: experience building and selling psychometrics; and a client list for psychometric targeting. Psychometrics is another targeting layer that would be sold in Google+ alongside dems targeting.

[1] http://www.mindset-media.com/marketers/profile/