My oldest was always introverted and quiet and a homebody
There is nothing wrong with that, imo
Sometimes I hear of all the "popular kids" going to parties all the time, and that sounds such fun, but then again, my DS is happy with his friends, they are just not "party crowd"
Modern society is all about outward success, being seen to have lots of fun all the time, having masses of friends, being outgoing and sharing all this "social success"'on social media 
I'd rather my DS has 3 good friends and is happy, than.himchading some ideal of being a cool kid who goes out all the time.
We should really value our kids for who they are. Your DD sounds lovely. Leave her be and stop checking up on how often she is intouch with friends.
Back off, there isn't a problem. Try to address your own insecurities without pushing them onto her. Think back, what were you like at that age? Why do you think she should be different from how she is? What is at the root of this?