Google is buying Silicon Valley startup Meebo to help expand its social networking service.

The acquisition announced will bring more tools to Google+, an alternative to Facebook's popular online hangout. Meebo started as a system for connecting people by instant message but has since built other communication features used by an audience of about 100 million web people in the US.

Both companies are based in Mountain View, Financial terms of the deal weren't disclosed.

"We are always looking for better ways to help users share content and connect with others across the Web, just as they do in real life," Google said in a statement. "With the Meebo team's expertise in social publisher tools, we believe they will be a great fit with the Google+ team."

Since its debut nearly a year ago, Google+ has attracted more than 170 million users. Despite that impressive growth, Google so far has had trouble persuading people to visit its social networking website as regularly as Facebook's more than 900 million users frequent its website.

Meebo works with publishers and advertisers to help them connect with web surfers for longer periods.

"Together with Google, we're super jazzed to roll up our sleeves and get cracking on even bigger and better ways to help users and website owners alike," Meebo wrote in a Monday blog post.

Meebo has raised $70 million in venture capital since it was founded in 2005 by Seth Sternberg, Elaine Wherry and Sandy Jen. Sternberg, who formerly helped IBM identify acquisition targets, served as Meebo's CEO.

Google has spent more than $16 billion buying 140 companies since the end of 2009. That includes the biggest deal in Google's 14-year history, a just-completed $12.5 billion acquisition of cellphone maker Motorola Mobility Holdings.

Google shares fell 59 cents after hours to $578. They ended regular trading up $7.61 at $578.59 before the deal was announced.