• by Mark Walsh  @markfwal, April 24, 2012

Pivotal Research Group lowered its valuation of Facebook to $75 billion from $82 billion a day after the social networking giant reported first-quarter results that showed rising costs and slowing growth since the prior quarter.

Facebook’s Q1 revenue of $1.06 billion was up 45% from a year ago but fell 6% from the fourth quarter in part because of a seasonal decline in ad sales. Net income in the first quarter dropped to $205 million from $302 million in the fourth quarter.

Calling the first-quarter numbers “disappointing,” Pivotal’s Brian Wieser said in a research note Tuesday said the results raise concerns around operating expense management at Facebook, especially given the overall diminishing growth during the quarter. The 45% year-over-year revenue gain fell below the research firm’s estimate of 51% for the quarter. It also reduced its estimate of operating margins by 3%.

The readjusted outlook lowered Facebook’s expected price per share at the time of its IPO to $29 from $32, for an overall valuation of $75 million. “We suspect much of the quarter’s results were due to investment against the core business, but that at least some of the acceleration will recur in near-term quarters,” wrote Wieser.