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A Facebook logo on a computer screen is seen through a magnifying glass.Reuters

Social networking giant Facebook said it would significantly increase its spending after having reported robust results for the third quarter, but its shares plunged in after hours trading amid investor concerns.

In a conference call with analysts, the company's chief financial officer David Wehner said its expenses would go up as much as 75% for the year, in line with its move to invest in both engineering and acquisitions.

He also forecast revenue growth would slow to between 40% and 47% in the fourth quarter from 59% in the third quarter.

"We believe that we have very substantial growth opportunities in front of us and we plan to invest aggressively to capitalise on those opportunities," Wehner said.

Facebook reported third-quarter revenues of $3.2bn (£1.98bn, €2.5bn), representing a 59% increase from last year. Revenue from advertising was $2.96bn, a 64% increase from the same quarter last year.

Mobile advertising revenue represented about 66% of advertising revenue for the third quarter of 2014, up from about 49% of advertising revenue in the third quarter of 2013.

Monthly active users of the social network increased 14% year-over-year to 1.35bn as of 30 September, with a 29% increase in users accessing the website via mobile phones.

Quarterly profit increased 90% to $806m, while adjusted profit increased 73% to $1.15bn.

Total costs and expenses increased 41% to $1.81bn for the third quarter. Recently, the company acquired messaging app WhatsApp and virtual reality headset maker Oculus Rift.

"This has been a good quarter with strong results," said Mark Zuckerberg, Facebook founder and CEO.

"We continue to focus on serving our community well and continue to invest in connecting the world over the next decade."

Nevertheless, investors seemed not happy with Facebook's aggressive spending plans. The company shares declined almost 10% in after hours trading.