UPDATE 1-Akamai revenue below estimates; shares fall

* 4th-quarter adj earnings $0.54/shr vs est $0.49

* Revenue $378 mln vs est $381.3 mln

* Shares down 13 percent in after-market trading

(Adds CEO comments and details; updates share movement)

Feb 6 (Reuters) - Internet content delivery company Akamai Technologies Inc reported quarterly revenue below analysts' estimates as online activity during the holiday season was not as strong as expected, sending its shares down 13 percent in after-hours trading.

The company's servers across the world carry content for clients ranging from Facebook Inc to Netflix Inc .

"(Online activity) was not as strong as in 2011. That difference accounted for revenue shifting a bit below the midpoint of our range and a little below the expected number on the Street," Akamai co-founder and Chief Executive Tom Leighton told Reuters.

The company's net income rose to $68.3 million, or 38 cents per share, in the fourth quarter, from $60 million, or 33 cents per share, a year earlier.

Excluding items, it earned 54 cents per share. Analysts on average had expected earnings of 49 cents per share, according to Thomson Reuters I/B/E/S.

Revenue rose 17 percent to $378 million, but was still below the average analyst estimate of $381.3 million.

Akamai, whose competitors include Level 3 Communications and Limelight Networks, helps companies, government agencies and other enterprises increase revenue and reduce costs by improving the performance, reliability and security of their internet-facing operations.

Shares of Akamai were down at $36.10 in extended trading. They closed at $41.58 on Wednesday on the Nasdaq.

(Reporting by Sruthi Ramakrishnan in Bangalore; Editing by Sriraj Kalluvila, Maju Samuel)