Although Sprint seems to be experiencing some success from its recent launch of the Samsung Instinct, the firm's increased investment in marketing doesn't seem to be paying off.  Sprint increased its 2007 ad spend by almost 5 percent to $1.313 billion, yet the firm's market share still slipped by 1.7 percent to 21.1 percent of the 255.4 million wireless subscribers in the U.S.  Article