Shares of Twitter Inc. posted their biggest drop in three weeks after analysts downgraded the stock, saying the company’s new advertising products may not be enough to warrant the stock’s climb since its initial public offering.

Twitter slid 4.1 percent to $56.61 Monday, for the steepest decline since Nov. 25. The stock has more than doubled since its IPO Nov. 6, leading Wells Fargo & Co. to recommend selling the shares and SunTrust Robinson Humphrey Inc. to advise holding them.

The company, whose service lets users post 140-character updates to friends and followers, Monday unveiled a new advertising format for marketers to boost their presence on mobile devices. In the weeks since its IPO, Twitter has also enhanced keyword and audience targeting and integrated an acquisition that will help with advertising on other mobile applications, boosting the stock price.

“Though we believe Twitter has emerged as a leading social communication channel, and is likely to continue a rapid release of improved advertising products and measurement capabilities, we believe investors underestimate some challenges facing the company and advertisers seeking to employ the platform,” Peter Stabler, an analyst at Wells Fargo, wrote in a research note Monday. He calculated that about half of Twitter’s monthly active user base hasn’t posted on the site in the past month and said television-marketing products may be limited in their reach.

Monday’s advertising update takes “promoted account” ads that normally run on the side of a Twitter timeline and puts them into a users’ stream of updates, the San Francisco-based company said Monday in a blog post. The change lets the promotions appear more prominently on mobile devices.

“We think all of these announcements are positive, but expected, and they don’t fully explain the recent stock run,” Robert Peck, an analyst at SunTrust Robinson, wrote in a note to investors. “The company is still in a very nascent stage of its life cycle.”