I'm happily single, my kids have really active social lives, my social life is my own.
I rarely go out on Friday evening through choice really, I am knackered after the working week. I do a yoga class at 6pm usually, then come home and make something to eat, might have a glass of wine if I am not picking up kids later on.
There are lots of things you can do, meetup is good if there is one near you. I go to a painting class once a week on a weeknight and one saturday a month. If I'm not seeing anyone I go to a local NT place for a walk and coffee/sandwich to get me out of the house. Might pop into a friendly neighbour or invite them round for a cup of tea. Visit my parents once a month (takes a while as the drive can be 90 mins in traffic). I also go to a meditation group now and then. I try to do a sport once a week which takes half a day, I don't always manage it but it is something I can just do if I am at a loose end.
A treat can be anything you want it to be, a walk into town to buy a candle and bar of chocolate/can of mixed G&T could cost as little as a fiver if you want it to. One of those foot mask things or a face mask could be as little as £1.
You're right, it is difficult sometimes when all of your friends are partnered up. You could just try one little thing each weekend and see if it makes a difference to how you feel.