IMHO the golden years for making friends are the school years, peaking in High School and College. After that it's a desert compared to that oasis.
That said there's a cheat I've heard of for the terminally lonely extravert: Clubs and Volunteering. I haven't tried either myself as I'm an introvert and near hermit happy with my three super-extravert friends that only need to hear from me a couple times a year.
Then there's the ultimate cheat, get married and have kids. Totally worth it, IMHO.
