NEW YORK — Following explosive entrances by Facebook and Twitter into the marketplace, as well as a strong performance from Google, U.S. mobile advertising spending grew 178 percent last year to $4.11 billion, according to a new forecast by eMarketer. Spending, the firm said, is expected to rise a further 77.3 percent to $7.29 billion in 2013.

That’s a nearly fivefold increase in spending since just 2011 and includes ad dollars spent on display, search and messaging-based formats sent to all mobile devices, including tablets. By 2017, eMarketer projects U.S. advertisers will devote $27.13 billion to mobile — just less than 45 percent of all digital ad spending and 13.8 percent of total media ad spending that year.

Based on year-end reporting from several major mobile publishers — along with estimates from other sources, including research firms and eMarketer interviews with ad agency executives — eMarketer revised its estimate for 2012 spending only slightly upward from the previous forecast in December 2012.

While top-line figures were relatively unchanged since the previous forecast, there were some small adjustments in market share and revenues for some of the larger players following Q4 results.

Google, thanks to its dominance of the mobile search market and strong showing in mobile display, is by far the largest player in the space with 93.3 percent of U.S. net mobile search ad dollars going to the company last year. Overall, more than half of total U.S. mobile ad revenues will go to Google this year, and its share will grow by nearly 3 percentage points by 2015.

eMarketer believes further mobile monetization of YouTube will contribute the lion’s share of incremental growth to Google’s mobile display revenues, while search ad revenues will continue to rise rapidly.

Facebook, the No. 2 mobile ad publisher in the country, accounted for 9.5 percent of mobile ad revenues in 2012 and is expected to take 13.2 percent this year. In the mobile display market, however, Facebook is on top, projected to grab nearly three in 10 dollars this year. eMarketer revised Facebook’s share of US mobile display advertising ad revenue upward by several percentage points after fourth quarter results came in higher than previously expected.

Google and Pandora round out the top three mobile display slots, with Twitter — whose figures were also significantly increased in the new forecast — making a strong push for the fourth spot. The microblogging service took in 7.3 percent of net U.S. mobile display ad revenues in 2012, the first year it offered such ads.

Both Facebook and Twitter have benefited from their use of so-called native ad formats that are seamlessly integrated within the core user experiences of their respective products. The resulting ability for both companies to deliver mobile ad impressions at much higher volume than many traditional ad publishers has helped them capture market share very quickly.

The most significant adjustment in the new forecast is a downward revision to Millennial Media’s mobile display ad revenues and market share for 2012 through 2015. The company’s traffic acquisition costs (TAC) paid to partners and ad publishers are not expected to fall in the next few years, as eMarketer previously believed. Millennial will take an estimated 2.8 percent of U.S. mobile display revenues this year, excluding TAC. Apple will come in slightly higher at 6.3 percent, eMarketer estimates.