I joined groups and gyms and from those I have met some of my best friends.
I joined a running group years ago to train for a 10K, then stayed on to do marathons and triathlons. When you are running for a couple of hours or more, its great to have a group to talk to. Now, I have a core group of 4-6 women who get together to run and compete.
I also have joined gyms over the years. Two guys that I started spinning with 10+ years ago bike with me over the summers. They are still in many of the classes that I go to and its easier to get there knowing I will see them.