Get out and socialize. This is a good time to meet to people. If you're one of those people that wanted to go somewhere but don't like leaving friends maybe this is a good time for a move. If you feel like you don't know anyone and are starting over is this the place you want to do it?
Idk. I've always talked to strangers ever since I was little so I'm always making new friends and strange acquaintances. Just talk to the people around you. The people at the bus stop, on the lift, in the park, the people at the bar. Not saying every one of those or even most of those will end in any kind of friendship but meeting new people ensures that there are a constant stream of people that could randomly end up being your best friend at any point.
Some of my best friends, actually most I've met totally randomly. I don't talk to anyone from my high school, really don't even hang out with that many people in the town I live but I'm out on road trips a lot and I've made friends all over.
I don't think I have any amazing friend making skills. Hell I'm somewhat socially retarded and have some LD action going on but I just talk to people. Some people are assholes, some people are awesome, you never know until you strike up a conversation.
This may seem weird to just talk to everyone , maybe it is, but I haven't gotten shot over it yet and it's help me meet some great people all over the place.
Best of luck. Don't dwell on the fact that people are gone. People will move in and out of your life over time. Don't worry so much about keeping friends as just living. If you talk to some strangers around you you'll find new friends that fit where you are in life now.
I missed some of the people in high school for a little bit but for the most part I was just friends with many of them because we were locked in a big box for 7 hours a day together. As time goes by your life changes as does other peoples. It may seem like a bad thing when you drift a part, sometimes it's hard but IMO it's a good thing in many ways. We're constantly changing as people. Your jobs might change, interests, anything. I'm not saying to push away your friends but if you drift apart sometimes it's because you're moving in 2 different directions. Sometimes it's worth the strain to keep those friendships but IMO many times it isn't. You just move on. Maybe keep their number and shoot em a text or call here and there. Visit them if you're out near them to catch up, but for the most part move on.
There are billions of people on this earth. That means there are a shit ton of cool people that could potentially be your best friend. Talk to the people around you. If nothing else, being nice and striking up some friendly conversations makes the world a little bit nicer.
Good luck OP. Just hang in there and  keep your head up.