Facebook Inc. (FB) on Thursday unveiled a new smartphone application that greets users with its mobile software each time they check their handsets, the social network's latest effort to grab a bigger share of the mobile market.

The software modifies Google Inc.'s (GOOG) ubiquitous Android operating system by showing a modified version of Facebook's mobile app on the home screen, making the service a gateway for phone calls, messages and a universe of other applications that users can download from Google's app marketplace.

"Today our phones are designed around apps and not people," Facebook Chief Executive Mark Zuckerberg said. "We want to flip that around."

The software will be available for download on Android phones on April 12, with a similar application coming for tablets in a few months.

Facebook, like its rivals, has increased efforts to grow its business on mobile devices as consumers spend more time accessing the Internet through smartphones. The Menlo Park, Calif., company reported 680 million active users of its mobile app last year, up 57% from 2011.

Analysts have long speculated about Facebook's interest in building its own handset, a possibility Mr. Zuckerberg downplayed last year as "the wrong strategy" for the Internet company. Instead, Facebook said it's partnered with Taiwan-based HTC Corp. to modify Android software on the kind of hardware it already builds.

Google develops Android under an open-source licensing model, allowing device makers to tweak the operating system to fit their products.

Write to Drew FitzGerald at andrew.fitzgerald@dowjones.com

Corrections & Amplifications

This item was corrected at 6:40 p.m. EDT to clarify that Facebook is based in Menlo Park, Calif. The original incorrectly stated Palo Alto.

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04-04-13 1357ET