Over the years I have developed a strategy for building a social life that works pretty well for me. I have followed it with varying rigor at different times, but I try to be strict about it particularly in the first couple of months of a new tour when I don't know anybody at all, and around summer rotation season when I find a lot of my friends have left. It's pretty simple; there are only two rules:
RULE 1: Accept all social invitations unless you have a Really Good Reason not to. This is self-explanatory, except for the Really Good Reason part. Here you have to be strict with yourself. Really Good Reasons include previous engagements, being out of town, actual illness, and for-realz emergencies. Just not feeling like going out is not a Really Good Reason. Does this mean sometimes I go out when I don't feel like it and have a lousy time? Yes, but rarely. Mostly I end up having fun, and getting over that initial hurdle of leaving the house is completely worth it.
RULE 2: Once a week, invite someone to do something social. A meal, a drink, a movie, a joint workout, a cultural event of some kind. Anything at all. They don't even have to say yes. The important thing is that you make the effort; the response is out of your control. Of course, when they do say yes, you are not allowed to bail unless you have a Really Good Reason (see Rule 1), so pick something you'll enjoy. A side benefit of this is that, knowing you have to invite someone to something every week, you are more likely to actively look for fun things to do in your town instead of just surrendering to Netflix and Chill.
The beauty of this system is that it's self-reinforcing: the people you invite to things under Rule 2 are more likely to invite you to things, which you accept under Rule 1, and where you may meet new nice people to invite to things under Rule 2, and so on and so forth. Is it foolproof? No. Maybe you're at a tiny post with security restrictions that make it hard to go places, do things, and meet people. In that case I'll give you some book recommendations. But after three moves in the FS and a whole bunch before that, I have learned that the single most important factor in making new friends is to TRY. Make an effort. Most of the time it pays off. You're good enough, you're smart enough, and doggone it, people like you!
And for those times when you need a little bit more encouragement, here is your anthem: