Britney Spears doesn’t turn 20 until Dec. 2, but she got an early birthday present this week. Her new album, “Britney,” sold 745,750 copies in its first week of release, becoming the second highest-debuting album of the year. (The No. 1-selling album record belongs to her Jive Records labelmates and boyfriend Justin Timberlake’s pop quintet ‘N Sync, whose “Celebrity,” released in July, sold 1.8 million copies its first week.) Spears’s 2000 album, “Oops! . . . I Did It Again,” debuted at No. 1 on 1.3 million units sold. Britney’s current achievement dethrones Michael Jackson, whose first album in six years, “Invincible,” debuted at No. 1 last week but now falls to No. 3 after selling 202,350 copies for the week ending Nov. 11, according to SoundScan figures. (No. 2 this week was Pink Floyd’s new compilation album, “Echoes — The Best of Pink Floyd,” which sold 214,650 copies.) Still, Jackson did all right for himself. The so-called King of Pop was crowned king of Nielsen on Tuesday. Jackson’s first domestic concert special in more than a decade, “Michael Jackson: 30th Anniversary Celebration,” raked in an average of 25.7 million viewers and a 10.9 rating/26 share in adults 18-49, dwarfing the competition for the night. The Hollywood Reporter notes that viewership of the special increased steadily throughout the 9-11 p.m. telecast, peaking to 29.8 million from 10:30-11 p.m. According to Nielsen Media Research estimates, 45 million viewers tuned in for all or part of the special.