Plaxo, the social address book service, has announced today that it signed an agreement to be acquired by former partner Comcast for an undisclosed sum.

Last Year, Comcast partnered with Plaxo among others, to provide its "SmartZone" centralized communications portal to Comcast subscribers, set to launch later this year. Building upon that collaboration, the cable company now plans to fully integrate Plaxo's properties -- which include social networking site Pulse -- into its own Web services Comcast.net, Fancast, and Fandango.

The ultimate plans for this collaboration are yet undetailed, but Plaxo's blog notes that "Comcast has an exciting vision to bring the social media experience to mainstream consumers...across all of the devices they use."

Citing things like the importance of media sharing to Comcast's customers, and referring to the service taking root in Comcast's portal, Web sites, "and the television," Plaxo's modest 50-employee outfit has already more than doubled its potential audience through this acquisition. Comcast's subscriber base exceeds 24 million customers.

The terms of the deal were not disclosed, but the acquisition has been independently valued at between $150-$170 million.