15 years ago I started cycling, by going to amateur XC races. I did that for almost 10 years and it was fun, I got some friends doing it and it worked great for a while. A combination of cycling injury and many people leaving the country ended that endeavor, but it is an example of some simple and practical approach. I was not even looking to socialize, but improve my sedentary life, I found others with similar interests.

I have a childhood friend that is cycling, but he lives in the Netherlands. There people who are cycling more than daily drivers are interested in athletic performance or activism and it kind of sucks, but he got friends skiing and scuba diving, as long as there are common interests you will find some decent people to socialize with.