When Bee does meet up with friends, she will put Olive down at her usual time (~7:30-8 or so) and then head out right away... then we will cross our fingers and hope that Olive doesn't wake up! I can almost never get Olive down in the evenings, so if Olive does wake up then I will text Bee and then walk Olive around and her room sing songs to her until Bee gets home (that's only happened once, and Bee was already on her way home at that point, thank goodness!).
If I meet up with friends, it's a bit easier because Charlie loves it when Bee puts him down. So she can put both kids down and I can head out before Olive is even asleep. I really almost never go out though, but it's an awesome option to have.
Going out together requires getting a babysitter, or family (usually Bee's mom). We've only gotten a babysitter a few times that I can remember. It's just so expensive!
How do you and your SO balance socializing with friends with parenthood?