كتب : Hatem Maher
Barcelona are on top of the league table with 33 points, four ahead of second-placed Real Madrid, who face Sevilla in a late Saturday kick-off.
The Brazilian playmaker had the first chance for the hosts in the 12th down when he received Edmilson's chip pass on his chest before sending a brilliant lob but Sociedad keeper Claudio Bravo tipped it over the bar.
Eight minutes later, Eidur Gudjohnsen latched on to Xavi's through pass before slotting home but his goal was ruled out for offside.
The offside flag denied Barca's opener again in the 23rd minute when Deco scored from the rebound after Ronaldinho's initial shot was blocked by Bravo.
Barcelona skipper Carles Puyol was replaced by Oleguer in the 27th minute after sustaining an injury.
Ronaldinho wanted to repeat the piece of magic which saw him score a stunning goal against Villarreal when his brilliant overhead kick in the 32nd minute from Ludovic Giuly's cross went just wide.
He finally broke Sociedad's resistance one minute past the hour mark after he connected with Giuly's cross to send a left-footed low drive into the bottom corner.
Xavi almost doubled the lead nine minutes from time when he was released by Ronaldinho's through pass but he hit the side netting.
Sociedad striker Darko Kovacevic wasted an opportunity to level the score one minute later when a defensive mix-up from Oleguer and Lilian Thuram slipped the ball to the Serbian, whose close-range effort was denied by Victor Valdes.
Barcelona played the last five minutes with 10 men after left-back Sylvinho was sent-off after collecting his second yellow card.