Please don'y get worried about it, your daughter will pick up on it and may feel that she's failing you because you want her to have more friends. As the others say, just support her, if you fret about it it will only make her anxious.
I would also advise you to talk to her teacher, see if she has noticed any nastiness towards your daughter or if she herself is concerned by her lack of socialising. My mum teaches this age range and she is very aware of what's going on because she knows that the little girls can be quite cruel with their, 'your my best friend and your not my friend' jibes. Your daughters teacher will probably be the same. If your daughter is upset by anything occuring at school the teachers should definitely be told.