NEW YORK ( The Deal) -- Shares of Millennial Media ( MM) tumbled more than 18% Wednesday, August 14, after the Baltimore-based company unexpectedly announced the acquisition of rival JumpTap to form a mobile advertising platform comparable to Google's ( GOOG).Under the mostly stock transaction, JumpTap shareholders will receive about 24.6 million shares of Millennial, or approximately 22.5% of outstanding shares, Millennial said August 13, after the close of regular trading. A cash consideration of slightly under $12 million will also be made to shareholders of JumpTap, the second-largest independent mobile advertising platform in the U.S. behind Millennial. According to a statement issued by Millennial, JumpTap's enterprise value is approximately $232 million, based upon Millennial's August 9 closing price of $9.13 and the $12 million cash consideration. "Long term, I think this makes sense because scale matters, and this brings scale," said James Cakmak, a Telsey Advisory Group analyst. "It makes them a much more formidable player. But there's a lot of questions that have surfaced in regards to timing and pricing and financial potential." According to International Data figures, the new entity will push Millennial's mobile advertising market share to about 28.7% from 18%, putting its share at a level comparable to powerhouse Google, Millennial CEO and co-founder Paul Palmieri said in a Tuesday earnings call with investors. Still, shares of Millennial, which trade on the New York Stock Exchange under the symbol MM, closed down 18.82%, to $6.90, on Wednesday. Clearly, the JumpTap acquisition didn't sit well, because on Tuesday, Millennial reported second-quarter revenue of $57 million, a 45% jump from $39.4 million for the same period a year earlier. Why the investor backlash? Cakmak explained that even though the scale advantages are clear, "the valuation for JumpTap could be slightly rich as compared to other ad-tech companies in the space." He also said investors may be disappointed because, despite the 45% rise in quarterly revenue, an even-higher increase -- to $59.23 million, according to consensus estimates -- was expected. The acquisition of Boston-based JumpTap is expected to close in the fourth quarter, subject to regulatory and shareholder approval. Upon completion, JumpTap CEO George Bell will join Millennial's board of directors as vice chairman.