The Sony release features Leighton Meester as a psycho freshman who becomes obsessed with her new roomie (Minka Kelly).
The 3-D underwater cave adventure "Sanctum," whose producers include "Avatar" creator James Cameron, drew modest crowds and came in second with $9.2 million, according to studio estimates Sunday.
Released by Universal, "Sanctum" is a survival story about explorers trapped underground in flooded caves during a monster storm.
The previous weekend's No. 1 movie, Anthony Hopkins' exorcism thriller "The Rite," fell steeply to sixth-place with $5.6 million. The Warner Bros. release raised its 10-day total to $23.7 million.
Revenue estimates were off sharply on Sunday as football fans gave movies a pass in favor of the big game. Even without the Super Bowl, though, it was another sleepy weekend at theaters, with overall business down for the 13th-straight weekend compared to last year, when Cameron's 3-D sensation "Avatar" still was riding high.
"I think Hollywood would be crying right now were it not for the 'Avatar' excuse. It really was an anomaly how well that film was doing in January and February last year," said Paul Dergarabedian, analyst for box-office tracker Hollywood.com.
This weekend's receipts came in at $86 million, down 25 percent from the same weekend in 2010, according to Hollywood.com. A year ago, the weekend was led by "Dear John" in first-place with $30.5 million and "Avatar" in second with $22.9 million, a huge number for a film that already had been in theaters for nearly two months.
Largely because of "Avatar," overall revenues so far this year are running 24 percent behind 2010's.
Still, Sony was happy with the results for "The Roommate," which nearly earned back its $16 million production budget over opening weekend.
Though trashed by critics, the movie gave room for a showy performance from "Gossip Girl" co-star Meester as the wacko roommate.
"She really goes for it," said Rory Bruer, Sony's head of distribution. "Crazy and beautiful. It kind of works for her."
"Sanctum" brought in spare change compared to the billions of "Avatar," though its opening weekend went a long way toward recouping the $12 million that Universal and partner Relativity Media paid for its distribution rights.
And the studio was happy to be in business with Cameron, a pioneer in digital 3-D production.
"Anybody working with this man, you have to revel in the opportunity," said Nikki Rocco, head of distribution for Universal.
While Hollywood is off to a slow start in 2011, the industry still could have a record year for revenue once a huge lineup of late-spring and summer blockbusters arrive, Dergarabedian said.
Last year was front-loaded with "Avatar," but business slowed through much of the summer and fall.
"The long shadow of 'Avatar' will start diminishing pretty soon," Dergarabedian said.
Estimated ticket sales for Friday through Sunday at U.S. and Canadian theaters, according to Hollywood.com. Final figures will be released Monday.
1. "The Roommate," $15.6 million.
2. "Sanctum," $9.2 million.
3. "No Strings Attached," $8.4 million.
4. "The King's Speech," $8.3 million.
5. "The Green Hornet," $6.1 million.
6. "The Rite," $5.6 million.
7. "The Mechanic," $5.4 million.
8. "True Grit," $4.8 million.
9. "The Dilemma," $3.5 million.
10. "Black Swan," $3.4 million.
Universal Pictures and Focus Features are owned by NBC Universal, a unit of Comcast Corp.; Sony Pictures, Sony Screen Gems and Sony Pictures Classics are units of Sony Corp.; Paramount and Paramount Vantage are divisions of Viacom Inc.; Disney's parent is The Walt Disney Co.; Miramax is a division of The Walt Disney Co.; 20th Century Fox, Fox Searchlight Pictures and Fox Atomic are owned by News Corp.; Warner Bros. and New Line are units of Time Warner Inc.; MGM is owned by a consortium of Providence Equity Partners, Texas Pacific Group, Sony Corp., Comcast Corp., DLJ Merchant Banking Partners and Quadrangle Group; Lionsgate is owned by Lions Gate Entertainment Corp.; IFC Films is owned by Rainbow Media Holdings, a subsidiary of Cablevision Systems Corp.; Rogue Pictures is owned by Relativity Media LLC; Overture Films is a subsidiary of Liberty Media Corp.