I have wondered the same thing. In my son's case, he has friends at school but when it comes to home, we don't live near kids his age. However, despite this, he does seem to easily make friends at school and or other places we go. Also, he has always seem to enjoy hanging out with adults. Overall, he is a happy kid. Fearing that my son doesn't have playmates at home, I have asked him from time to time, how do you feel about another sibling?? his reply, " I told you I want to remain single." Basically, he likes being the center of attention :) and while he does make friends pretty easily and has forged relationships at school (this despite having to attend a new school last year at the beginning of 5th grade) he is well-adjusted... He's the type of kid that enjoys busying himself with projects, e.g. he will play in his room for hours making maps, playing his accordion and or drawing.. He has never been one to have to be with a lot of people .. Although when he is, he does ok..
Bottomline, I think if a child seems happy , then so far, I wouldn't worry too much. I would just keep the lines of communication open (that's what I do with my son) ask about her day and see how first grade unfolds... sometimes, kids just get off to a slower start.. don't forget, she now has more work than in kinder.. could be that is overwhelming and making friends is now secondary..