MENLO PARK, Calif. — At Facebook headquarters, mobile devices are everywhere. The first thing visitors do is sign in on iPads. Employees at the cafe use iPads as cash registers. Instead­ of sending e-mails, employees use the company’s messaging system. Messages pop up on both mobile devices and PCs.

Mobile is clearly on the company’s mind, especially now that people are spending more time using Facebook on mobile apps than on computers.

And on Wall Street, investors are putting pressure on Mark Zuckerberg’s social network to master the mobile world so it can speed up growth and lift its sagging stock.

But the challenge for Facebook, along with other companies like Google that got their start on the Web, is figuring out the best way to serve lucrative ads to mobile users without cluttering up their small screens. Facebook’s executives say the company is diving deep into mobile, starting with new versions of its apps for the iPhone and iPad, which it released Thursday.

Users had complained the apps were sluggish; more than half of those who have rated ­Facebook’s iPhone app in the Apple App Store gave it one star out of five.

The new apps are faster because they were rewritten in the native programming language of Apple’s devices, replacing most of the Web-based technology used in previous versions.

The apps are part of what Facebook executives say is a transformation into a ‘‘mobile-first’’ company. Developing mobile products has been made a priority, they said in recent interviews.

‘‘We have basically retooled and focused the company around mobile,’’ said Mike Schroepfer, vice president for engineering. ‘‘It’s been a huge change.’’

Product teams have been arranged so that they make mobile versions of new features at the same time they are developed for the main website.

Facebook is also trying to spread mobile expertise throughout the company. Top engineers hold training sessions every week for 20 employees at a time, teaching them how to program for Apple and Android devices. About 100 engineers are now working on Facebook’s mobile products, said Cory Ondrejka, chief of mobile engineering.

With the training, the company expects to have created 200 new mobile engineers by the end of the year. Soon these classes will be open to any Facebook employee who wants to attend.

When Facebook filed for its initial public offering of stock in February, it listed the rise of mobile as a potential risk, because it had not yet discovered a clear strategy for making money on phones and tablets.

Gokul Rajaram, product director of ads, said Facebook’s strategy would mostly focus on so-called sponsored stories, which treat posts from users as ads, amplifying word of mouth. For example, if a user clicked the Like button on the Facebook page of a certain band, some of his friends might see a notice about this when they visit the site. But if that band chose to sponsor that Like, the notice would show up on most of the friends’ news feeds, on both mobile devices and the website.

Facebook started using sponsored stories in February, which now generate about $1 million of revenue a day.