Facebook is planning acquisitions in the next four months that will improve the site’s design, keep its service reliable and advance mobile features. The company aims to make about 20 purchases in 2011, up from 10 last year and one in 2009, Vaughan Smith, Facebook’s director of corporate development, said in an interview granted to Bloomberg.

Facebook 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.”