There are all kinds of hiking clubs...if you are in Southern California, there are just a ton of them...very easy way to meet new friends with common interests. Just spend some time "googling" and you will find some good ones.

Even here in Atlanta, there are at least three major outdoors-oriiented social organizations that I'm aware of and probably some smaller ones too. If you are in Kansas or some place like that, it might be more difficult, but any major city even remotely near some mtns will have a big outdoor enthusiast community to tap into.

If there is a big sporting good store close to you (like REI), you can ask the people there and may know of some, but I say mostly just look online.