Jenny, I've totally experienced the same thing. I am 28 and have one friend left from high school/junior high (although he's male), and one friend left from college. I talk to other college friends sporadically, but nothing regular. I also felt at your age that I had much different priorities than others my age. I totally overdid the going out drinking thing in college, so after college when other people my age were going out on weekends, I had no interest in that. I also found a job working with juveniles right out of college that required a lot more hours than my friends and was promoted quickly to a job with a LOT of 24/7 responsiblity. So while my friends were still in that carefree college mode I was responsible for a 24/7 residential facility. And I met my husband at 23 (we just got married in September). So while my old friends were out partying, I was working or at home with my future husband.
At this point in my life I have a very small group of close friends. One was a former neighbor (my husband is also very good friends with her husband) and one is married to my husband's best friend. Before I became good friends with them I definitely felt I was lacking friends, but I'm very happy with my small group of good friends.
I completely agree with what everyone is saying about age. Some of the people I've become closest to are 25 years older than me. I feel I can relate to them better than a lot of people my age. I have friends who are 26 to those in their 50's and 60.
I was glad you posted this question. It helped me to realize I'm not the only one who's experienced this.
Also, I've met some great people at the Y and I'm also taking a hip hop class with people my age. Another thing is to maybe do things with your boyfriend and other couples. You can get to know some great women that way while doing things as a couple.