Facebook Inc., the world’s largest social network, is planning acquisitions that will improve site design, keep its service reliable and advance mobile features to stave off competition from Google Inc. and Twitter Inc., Bloomberg reported.

The company aims to make about 20 purchases in 2011, up from 10 last year and one in 2009, said Vaughan Smith, Facebook’s director of corporate development.

Facebook is betting that a focus on design will entice people to spend more time on the site, and adding mobile services can cater to the growing number of members using handheld devices. The company has made 13 acquisitions so far this year, including adding a mobile group-messaging service it rolled out to users this month.

“Two years ago, we didn’t have a track record in acquisitions,” Smith said. “While we expected them to work well, it was still a crapshoot how they’d turn out. We’ve built a culture that supports entrepreneurs, and it’s working incredibly well.”

Facebook has made mostly small acquisitions, with target companies sometimes having one or two employees.