If your plan is simply calorie counting, then you really shouldn't be beating yourself up over this. Hummus and pita is a good snack to have, and probably wasn't any higher than the snack you would have had at home, especially shared with friends.
Part of learning to view this as a lifestyle change, and not a temporary "diet" is cutting yourself some slack when it comes to this kind of thing. You made a great choice given the situation, it's not like you ordered french fries and chocolate cake!
It's one small shared appetizer at one meal, just take it as a learning experience and move on. I make "mistakes" like this (actually much worse than this) several times a week... yesterday it was eating 1.5 pounds of grapes (oops), today it was surprise chinese food at work. I do what I can, walk and run to give myself a "buffer" against these challenges, and move on. I'm not going to let it get to me; I'm in this for the long haul.