Elsewhere, Cristiano Ronaldo became the first player to hit 30 La Liga goals in six consecutive seasons as he inspired Real Madrid to a 4-0 thrashing of Eibar while Fernando Torres was on target once more as Atletico Madrid triumphed 3-1 at Espanyol.
Winger Mikel Oiarzabal headed the only goal for Sociedad in the fifth minute.
"The game began in the worst way possible," said Barca coach Luis Enrique. "We played really badly in the first half.
"We didn't deserve to lose but we are still the leaders. Now we have to pick ourselves up after leaving this great stadium where we never do well."
Barca, without top scorer Luis Suarez through suspension, fielded a much-changed side and managed just five shots on target all game.
Luis Enrique's team have 76 points with six games to go, one ahead of Atletico and four in front of Real Madrid.
After going unbeaten in all competition for six months, Barca have followed last week's 2-1 defeat by Real Madrid with another slip-up.
The European champions are without a victory in three league outings, their longest stretch since Luis Enrique took charge in 2014.
The coach made four changes to the team that lost to Real Madrid, bringing back midfielder Rafinha after a six-month injury absence and starting without Andres Iniesta, Jordi Alba and Ivan Rakitic.
Barca were handed their fifth defeat in six visits to Sociedad after 18-year-old Oiarzabal nodded in a cross from Xabi Prieto.
The visitors struggled to get back in the game and home goalkeeper Geronimo Rulli did well to save from Lionel Messi twice, substitute Iniesta and Arda Turan.
Ronaldo netted for Real Madrid in the 19th minute and also provided passes for Lucas Vazquez and Jese to score as the hosts stormed into a 4-0 halftime lead against Eibar.
James Rodriguez had opened Real's account with a curling free kick in the fifth minute.
EIGHT CHANGES
Coach Zinedine Zidane made eight changes to the team that suffered a shock 2-0 defeat at VfL Wolfsburg in the Champions League in midweek with only Ronaldo, Pepe and Casemiro keeping their places.
Brazilian right back Danilo was booed by the home fans for his disappointing display at Wolfsburg when he came on in the second half but it was an otherwise positive afternoon for Real who host Wolfsburg in their quarter-final second leg on Tuesday.
Rodriguez was criticised for being caught laughing on the sidelines in Germany in midweek but the Colombia international lifted the crowd by scoring early on, his first goal in just under two months.
A fine piece of skill from Ronaldo allowed Vazquez to pass the ball into an empty net in the 18th minute before the record-breaker finished off a cut-back from the lively Jese a minute later.
Ronaldo returned the favour for the 23-year-old in the 39th minute as Real made it 4-0.
Eibar's top scorer Borja Baston hit the crossbar in a rare attack for the visitors who are 10th in the table.
Torres bounced back from his sending-off in the midweek Champions League defeat to Barca by scoring his third goal in as many games, cancelling out Papa Diop's opener in the 29th minute for Espanyol.
France international Antoine Griezmann grabbed his 20th league goal of the season to put the visitors ahead on 58 minutes and midfielder Koke sealed the points for Diego Simeone's side with a close-range finish in the 89th.
Simeone made one change from the team that lost 2-1 at the Nou Camp, replacing Saul Niguez with Augusto Fernandez.
Torres hit the bar early on but Senegal midfielder Diop headed Espanyol in front from a corner.
The former Chelsea and Liverpool striker equalised by converting a pass from Koke.
Torres was then also involved in the other goals by Griezmann and Koke.
(Editing by Tony Jimenez)
source: www.abs-cbnnews.com
