It’s kind of funny that business etiquette has moved much more to scheduled meetings even for short discussions, and social life has moved in the opposite direction.
replyAt higher levels, I think impromptu calls/messages of a time-sensitive nature are probably more common. But, in general, phone calls out of the blue are less accepted than they were 10-20 years ago outside of a very close circle. And in business there would probably be a preceding message to the effect of “can we chat?”
replyIt depends. My friends with kids have everything planned out months in advance. If they're to come out to something they have to have it all scheduled between judo classes and school birthday parties blah blah
replyThe rest of us just wing it. Which I really prefer. I hate having plans. Especially in case I might not feel like it on the night in question.