I know that Barcelona played the worst match of the season. But you are underestimating the mental factors in this game very much. If Barcelona got ahead then I think there would be a different game than we saw in the field. As a result this match is left behind and the thing Barcelona can do now is to learn their lesson from this match and focus on their upcoming matches.
What happened to Barcelona in the game against Real sociedad was never expected, I'm not surprise of it because football is full of surprises and sometimes we can't predict what the outcome of a game would be. Sociedad is just lucky to win the game , this lose is what Barcelona would learn from. Barcelona still remains the best team in laliga for this season and this lose will be a motivation for them to get more points.