Plaxo launched in 2001 as an before anyone had heard of social media.
Last year, it launched its own social networking site Plaxo Pulse as a rival to , although the site is more like in style.
However, the relaunch still presented Plaxo with an uphill challenge because of its lingering negative reputation for being a prolific, if accidental, spammer.
When first launched, Plaxo included a feature that would spam a users entire address book with offers to sign up to Plaxo. This "feature" was later turned off, but not before it had done a lot damage to the firm's reputation.
It is thought that Comcast will use Plaxo to build its own social networking offering to bring together its various online, email and mobile services. These include its video entertainment site Fancast, movie site Fandango and video publishing company thePlatform.
Sam Schwartz, executive vice-president of Comcast Interactive Media, said: "Every social network is better the more end points you can connect with.
"Plaxo will offer users the ability to share with friends on all Comcast screens going forward."
, CEO of Plaxo, said: "We intend to deliver on a vision of making social media a natural part of the lives of regular people, not just early-adopters."
Google and Facebook have been linked to Plaxo as potential suitors, but in the end both firms declined to make a bid when the social media firm touted itself around Silicon Valley.
Like Google, Plaxo was founded by two Stanford engineering students, Todd Masonis and Cameron Ring. Also on board early was Napster co-founder Sean Parker, but he was forced out only to re-emerge as the founding president of Facebook.
Plaxo is a supporter of the cross-platform social media alliance OpenSocial founded by Google and MySpace, and latterly joined by Yahoo!.
It has also forged partnerships with other online firms, including Microsoft, Google, Mozilla and Yahoo!, as it strived to synch address books and calendars.
Plaxo now claims to have 20m members, a rise of 5m from three years ago, in part because of a partnership with AOL.
The two companies estimate that tying Plaxo with Comcast could create another 30m users. If true, it would represent a remarkable turnaround for Plaxo and make the $175m maximum price tag seem relatively cheap.