Despite losing 12% of its young-adult audience compared with the previous year, USA reigned as the top-rated ad-supported cable entertainment network for the first three months of 2010 — its 15th consecutive quarterly victory.

According to Nielsen, USA averaged 3.1 million primetime viewers overall (down 4% from the first quarter of 2009 but roughly a third more viewers than its closest competitor this year), and 1.3 million in 18-49. It also led the cable pack in adults 25-54 and adults 18-34.

“Burn Notice,” averaging 3.2 million viewers in 18-49 and 6.35 million overall, gave USA cable’s top-rated ongoing primetime series, while “White Collar” and “Psych” saw bumps after moving to new nights.

Second behind USA in total viewers was Fox News Channel, which averaged 2.3 million in primetime. FNC showed a 16% year-to-year increase among adults 25-54 (ranking 12th among all cablers), while rival newsies MSNBC and CNN tumbled by 22% and 42%, respectively.

“The O’Reilly Factor” rose 28% in the demo to reach nearly 1 million viewers, and the 25-54 aud for “Glenn Beck” (a late afternoon show that’s no doubt aiding the net’s primetime) doubled to 731,000.

TNT took the bronze medal among basic cablers in the first quarter of 2010 with 2.1 million overall primetime viewers on average.

History (1.4 million viewers overall, 713,000 in 18-49) had its best quarter ever, driven by “Pawn Stars” becoming cable’s most-watched series in the 25-54 demo (2.6 million viewers). In addition, “American Pickers” was the No. 1 new series in that demo (1.9 million).

MTV’s “Jersey Shore” had the biggest 18-49 aud for any single episode of a series, drawing 3.8 million in the demo on Jan. 21. Tops in overall audience for an episode of a series was Nick’s Jan. 18 “iCarly,” which nabbed 12.4 million, the largest cable audience since the Dec. 28 Minnesota-Chicago NFL game on ESPN.

Powered by “iCarly,” Nick had its best quarter ever, and Nick at Night showed a 7% gain in overall audience compared to the year-earlier quarter.

Other cablers trumpeting record ratings were A&E, which grew 5% in 18-49 to 756,000 and 7% overall to 1.6 million, and Oxygen, up 11% in 18-49 to 271,000.

ABC Family increased its overall aud to an all-time level (1.5 million), though the cabler’s 18-49 numbers dipped slightly. TruTV stepped up 15% in 18-49 to 602,000, and E! was up 23% in 18-49 to 448,000. E! stalwart “Keeping Up With the Kardashians” doubled its previous aud by averaging 3.7 million overall viewers.

TBS dominated in cable sitcoms with four of the five top-rated comedies in 18-49, led by “Tyler Perry’s Meet the Browns.” Also boosted by repeats of “The Office,” “Friends” and “Seinfeld,” nearly caught USA as the quarter’s top cabler in 18-34.