Facebook Inc. and Twitter Inc. shares set new highs Monday, buoyed by growing optimism that social-media advertising will grab a bigger slice of the market for digital promotions.

Facebook, owner of the world’s most popular social network, ended up 4.8 percent at $57.77, after trading as high as $58.32. Twitter, the microblogging service with more than 230 million users, gained 7.7 percent to $64.64, after trading as high as $64.95. Facebook, which held its initial public offering in May 2012, has more than doubled this year, and Twitter has done the same since its IPO last month.

As marketers shift spending to social-media services, both companies will expand their share of the digital-ad market, according to EMarketer Inc. By 2015, Facebook will grab an estimated 9 percent, up from 5.9 percent in 2012, while Twitter may take 2.2 percent, up from 0.6 percent, the researcher said. Youssef Squali, an analyst at Cantor Fitzgerald, Monday raised his price target for Facebook to $65, saying ad revenue will increase as the company devotes more space to paid promotions.

“Ad intensity has increased throughout the all-important holiday period,” with more high-priced video ads mixed in, Squali wrote in a note to clients.

Facebook has about doubled the percentage of advertising it’s allowing in users’ feeds, similar to what happened during last year’s holiday shopping season, Squali wrote. The increase, which comes after the company said in October that it was holding back on the number of promotions, is likely to boost mobile and desktop advertising revenue. Menlo Park, California-based Facebook also began testing auto-play video ads last week, an offering that should be popular with marketers, he wrote.

Advertising Options

Twitter has been boosting its own advertising options. Earlier this month, the company said it would help sell native advertisements on mobile applications, a feature that blends promotions into the content around them. Twitter also expanded a tool that lets marketers target consumers who have already expressed interest in their products based on Web-browsing histories.

Including Monday’s gain, Twitter’s market capitalization has risen to more than $35 billion, even as the company remains unprofitable. San Francisco-based Twitter is now more valuable than Salesforce.com Inc., the top maker of customer-management software; food supplier General Mills Inc.; and Deere & Co., the largest maker of agricultural equipment.