I've found as I get older that I have little or no interest in making new friendships...or even maintaining some of the older friendships. I haven't met anyone so interesting I need to see them regularly...once or twice is plenty.
Identify those who need or want you as a friend.
Often, the older people become, more independent they become and the less disposable time they have to do things with friends. This is especially true when people get married and have kids. I am married with four kids under 13. I don't have any "friends" that I regularly do things with. No time. Once in a while we hang out with a neighbor to watch a Laker game but only because we both have kids in the same school and this guy and I used to work together.
I disagree with a previous poster that mentioned like minded people. You only have to have one thing in common. As I said it is not high school. You will have different friends for different activities.
It's a matter of getting out of your shell and knowing who to meet, really.
I've had a similar thing happen to me. Within the last year, a lot of my friends had either left the area, or have died. I started getting a little concerned about things. I have realized that there were more people that I could meet than I realized. So I started paying more attention to them.
All of a sudden, people started going out of their way to meet me. All of a sudden, I was overwhelmed with new people that I have met. Now I'm back in the pass(ive)enger seat.