Interesting question! I've never thought about this before so don't have a very reasoned response.

My initial reaction is that me-time is something where I'm not having to take any other needs into account, I am pursuing my own ends with pure selfishness. This is different from social engagements for me -- even though my social engagements are pleasurable, I'm always working to take the other person's needs into account. I want them to get something out of it so I work for that. The nicest social interactions require very little work, but it's still there, always.

Me-time is pure. The most usual example would be reading weird shit that nobody else cares about and that has no utility other than to interest me. The "no utility" part matters, because a lot of time I try to make the time spent useful to my larger life. The me-time doesn't need to be useful. It is an indulgence.