Meeting new people and making good friends gets harder as you get older. You get less adventurous, fall into comfortable routines with significant others, and you don't have school to force you to interact with different groups of people any more. But if you have at least one friend, you do have an easy option for finding some fresh faces to spend time with.
When we think of our friends, we tend to think of them as just our friends. Obviously, this is rarely the case. Your friends have friends of their own. If you're feeling lonely, or you feel like you don't have enough friends in your inner circle, Amie Harwick, licensed marriage and family therapist, suggests at The Cut that you should use the friends you have as a resource to network. Ask your current friends for friend referrals, as if they're a good doctor you trust to know what's best for you.
Think about it - the things you have in common with your current friend are probably some of the same things they have in common with their other friends. Have them set you up with someone they know that you don't (your friend can come along if you're uncomfortable). It might not work out - sometimes friend groups stay separate for a reason - but you could hit it off and have a new buddy just like that.
This approach worked extremely well for me when I moved to a new city several years ago. I met a someone through an old friend I'd known for a very long time, and we eventually grew close enough to become travel buddies - all because I asked my friend, "Hey, do you have so-and-so's number? He seems cool." Give it a shot. Worst case scenario you're left with the friend you already had.