After months of rumors, Plaxo, the social contact service, finally announced today that it is, in fact, being purchased — by Comcast.

Aside from Comcast, both Facebook and Google were rumored to be purchasing the service at one point. Of all of those, Comcast may seem to be the least likely suitor, but thanks to a deal the two already had in place, Comcast apparently thought it was time to take the relationship to the next step.

And that next step is an expensive one for Comcast. While the terms of the deal were not disclosed, TechCrunch is pegging the number between $150 and $170 million. Previous reports had suggested the company was looking to sell for between $100 and $200 million, so this sounds about right.

If those figures are true, it’s a win for investors, who pumped in slightly more than $20 million into the company. Sequoia Capital was a lead investor, and together with angel investor Ram Shriram, Sequoia helped orchestrate a change in course at Plaxo five years ago that led to the ouster of co-founder Sean Parker. Parker, who long had a vision to make Plaxo a widespread network upon which other services could be build, went on to help build Facebook. Plaxo never did fully realize that earlier, grander vision, but Plaxo did maintain a solid contact update service. After several years of relative silence, it emerged more recently to make significant strides.

Other Plaxo investors included Globespan Capital Partners, Harbinger Venture Management, Cisco Systems.

Mountain View, Calif.-based Plaxo was founded in 2001 and at first it focused on creating technology for syncing contacts across email applications. But the company angered many by sending non-solicited emails to users.

It restored some of its luster when it launched a new service called “Pulse” last year. Pulse is basically the equivalent of Facebook’s New Feed, except you can use it to track what people do on sites around the web — if somebody you’re connected to on Plaxo uploads photos to Flickr, you might see those photos in your Pulse stream.

As anyone who covers social media has seen, these feeds are all the rage nowadays, with Pulse’s biggest rival (besides Facebook and other social networks) being Friendfeed.

The company has also become an outspoken advocate of “data portability,” a geek movement that is bringing together social networks and other tech companies, to create new ways of sharing user data between them.

With Comcast, Plaxo promises that this deal will help its quest to make the web a more social place:

Joining forces with Comcast is a real win for our customers, our investors, and our employees. Comcast has an exciting vision to bring the social media experience to mainstream consumers. Together, we will be able to help users connect with all the people they care about, across all of the devices they use, with all the media they love to consume, create, and share. This is also great news for the Internet industry at large, where Plaxo has been – and will continue to be – a strong advocate for opening up the Social Web.

Also, I have Comcast coming tomorrow to install my cable. I wonder if they’re going to pitch Plaxo to me somehow? Or maybe I’ll just get an email about it.