Comcast Corp. said that it reached an agreement to Plaxo Inc., which operates an online address book and a social network called Pulse and provided the basis for other popular social networking sites, including Facebook and Friendster.

Financial terms of the agreement were not disclosed. Plaxo, which employs 50, will operate as an independent unit and remain based in Mountain View, Calif.

Plaxo said it already hosts the online address books of Comcast Web mail users and will work with Comcast to bring social networking capabilities to the portal Comcast operates for its high-speed Internet customers.

Plaxo is now developing technology that allows people to share photos on different electronic devices, including cable set-top boxes.

Every social network is better the more end points you can connect with, said Sam Schwartz, executive vice president of Comcast Interactive Media. Plaxo will offer users the ability to share with friends on all Comcast screens going forward.

Comcast will use Plaxo to offer social network links across Comcast-connected devices from TVs to digital video recorders to, eventually, wireless devices. This is due to a new partnership with Sprint, Clearwire and Google.

Plaxo joins Comcast's fast-growing unit which includes, video entertainment site Fancast, movie site Fandango and video publishing company thePlatform.