BlackBerry maker Research In Motion has lost its second marketing executive in four months, highlighting the difficulties RIM faces in keeping pace with innovative market leader Apple.

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 departure brings into sharp focus the pressure on co-CEO Jim Balsillie, who took on more promotional duties after marketing head Keith Pardy left RIM in March, just weeks before RIM launched its PlayBook wannabe competitor to Apple's hugely successful iPad.

RIM disappointed analysts with the PlayBook's launch, amid criticism that it had rushed the tablet computer to market before it was ready.

It disapointed them again last week with dismal results and a weaker-than-expected forecast for full-year results, prompting downgrades and 20 percent stock price slump.

The shares fell a further 6 percent on Monday, and have now have lost half their value this year.

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

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

RIM could see more staff defections in coming months after it said it would cut jobs in a bid to get back into the smartphone race dominated by Apple's iPhone and Android-based devices.

Trade publication Advertising Age said Wallace will become Samsung's vice-president of strategic marketing for the United States, working from 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 Janet Guttsman)