Not exactly young adult but I'm 35 and see friends almost weekly. If not in person I at least chat/voice chat with some of them.
I don't have kids, currently dating someone with 2 boys though. I don't take any overtime at work but I have online classes at night so I plan on dropping my hours by half. Also, I don't bring any work home with me. I separate the 2, religiously. (I am unionized, makes it easier to do so)
I live in a small city, far from any metropolitan area, people are so much more friendlier here.
IMO, it's all about hobbies. My hobbies align with most of my friends and I have more than one circle of friends.
I moved over 20 times in my life, different cities and all. So I know how to make new friends, I honestly think it's a skill. At the same time, friendship goes both ways, if you're the only one putting the work on the friendship, it's not really gonna work.
I have friends working on their PhDs and even post-PhDs (or whatever it's called) and they usually have time to see me / friends 3 or 4 times a month.
So yeah, I have a fulfilling social life, it's not like that all year round mind you, things change and people need breaks and what not sometimes.