Barca trail leaders Real Madrid by four points after stumbling to a 0-0 draw at home to Malaga on Saturday.
Goals from veteran captain Xabi Prieto, David Zurutuza and Inigo Martinez secured a comfortable victory for La Real, who leapfrog Atletico Madrid after their 3-0 derby defeat to Real Madrid on Saturday.
"We are in great form and confidence is high," said Real Sociedad boss Eusebio, a former Barcelona B coach, who could come back to haunt his former employers next weekend.
Barca trail leaders Real Madrid by four points after stumbling to a 0-0 draw at home to Malaga on Saturday and have lost all their of their trips to Anoeta in the last three seasons.
Villarreal remain above the San Sebastian side in fourth only on goal difference as they lost 1-0 to Athletic Bilbao.
Raul Garcia bundled home the winner midway through the second half to move Athletic up to seventh and just two points off the top four.
Valencia's struggles continued as they remain just two points above the relegation zone after a disappointing 1-1 draw at home to winless Granada.
Mehdi Carcela fired Granada into a deserved half-time lead in first-half stoppage time.
Nani levelled for Valencia just two minutes after the break, but despite dominating thereafter Valencia's winless run stretched to four games.
"We have hit rock bottom in every sense," said Valencia midfielder Enzo Perez.
"We have to be strong at home and we are not managing that."
In Sunday's lunchtime kick-off, Gerard Moreno made amends for missing a penalty with a much-needed winner for Espanyol in a 1-0 win at Alaves.