There's no easy line, but taking money out of your friend's pocket (drinking their alcohol, burning their gas) is shitty behaviour. I'd rather crimp something in my own budget. For instance, I went to a party this weekend, and brought alcohol. That alcohol would have been what I'd have had at home, so I just won't have alcohol at home for a month or so. Was it worth it? Absolutely. Great party, great friends, great memory--much better than having a drink while I skim tumblr.
In fact, I think I'd say that's my basic line: frugality is being cheap to yourself, cheap is being cheap to others.
I think everyone on this forum is generally okay with the basic concept of being cheap to yourself when there's a reason for it (to be able to retire early, to be able to buy more stock in a down market, etc), and most of us are not so okay with being cheap in a way that impacts our friends/family/people in need (never reciprocating favours, going to the food bank when you have a $500K stash). We draw each of those lines in different places, but I'd say that's my basic distinction.