Research In Motion lost a second marketing executive and its shares dropped 7 percent on Monday, part of a tide of bad news that has halved the BlackBerry maker's market value this year.

Brian Wallace, who headed RIM's digital marketing efforts, has taken a job at Samsung, one of the companies that is stealing RIM's smartphone market share.

The second departure in four months highlights the difficulties RIM is having keeping pace with innovative market leader Apple and an army of handset makers using Google's Android platform.

More defections over the coming months are likely after RIM announced it would cut an unspecified number of jobs as it seeks to regain its stride in a smartphone race that it once dominated.

The exit brings into sharper focus the pressure on co-CEO Jim Balsillie. He took on more promotional duties after marketing head Keith Pardy left RIM in March, just weeks before the launch of the PlayBook, RIM's disappointing challenger to Apple's hugely successful iPad.

The PlayBook launch was criticized by many analysts who said the company rushed the tablet computer to market before it was ready.

The company disappointed them again last week by revealing dismal quarterly results and slashing its forecast for full-year results. That prompted analysts to downgrade the stock and its shares slumped 20 percent.

The shares fell a further 6.7 percent on Monday -- closing at $25.89 on the Nasdaq -- and have now have lost more than half their value this year.

RIM also faces pressure to change its unusual management structure. Balsillie and co-founder Mike Lazaridis share the jobs of CEO and chairman of the board.

Shareholders vote next month on whether the two should retain their shared roles, following a push by a disgruntled investor.

Trade publication Advertising Age said Wallace will become Samsung's vice-president of strategic marketing for the United States, based in Dallas.

Reuters confirmed his exit with a source at RIM, while Wallace also acknowledged the move on his Twitter page.

The change comes at a pivotal time for Waterloo, Ontario-based RIM as it works to push out new smartphones with the QNX operating system that it uses in its PlayBook tablet.

Last week RIM said its chief operations officer, Don Morrison, would take a medical leave of absence. He is expected to return later in the year.

Adding to the woes, third-party Twitter client Seesmic said it would no longer support the BlackBerry platform, and would instead focus on Android, Apple's iOS and Microsoft Windows Phone 7.

(Reporting by Alastair Sharp; editing by Frank McGurty)