Google (GOOG) is expected to account for 46.8% of all that spending; that figure is slightly below 2013's 49.3%.
An ongoing challenge for Google: Web browsing accounts for only ~20% of smartphone usage, making Google's search ad hegemony less valuable on mobile than on PCs. Helping to offset: Google's AdMob unit remains a top mobile display ad player, and the company is making efforts to integrate mobile app content within search results.
eMarketer recently predicted mobile search would account for 26.7% of Google's 2014 U.S. ad revenue, up from 19.4% in 2013. Altogether, the firm sees Google's U.S. PC search revenue falling $770M this year, and its mobile search revenue growing $1.76B.
Facebook (FB), which saw its mobile ad sales rise 4x Y/Y in Q4 and make up 53% of its ad revenue, is expected to have a 21.7% mobile ad share, up from 17.5% in 2013.
Twitter's (TWTR - 75% of Q4 ad sales from mobile) shares is expected to rise to 2.6% from 2.4%. Pandora's (P - 72% of Q4 ad revenue from mobile) is seen falling to 1.7% from 2.1%, and Millennial Media's (MM - just expanded by merging with rival Jumptap) to 0.7% from 0.8%.