Thanks for all the comments, folks. I think this is an issue we all think about and have our own awkward moments over from time to time.
Leila, for the "breakup" moment, I find that what works best for me (when I remember to be graceful) is to say simply, "it's been great talking with you...and I want to do a little bit more networking before the end of the night." Sometimes I make a bit of a joke at my own expense or laugh about the process/structure of networking, like, "well, I really should get back on the horse and meet a couple more people."
Perhaps that's not the most grace-filled way to say it, but it's authentic for me to acknowledge that networking takes effort, and that while I'm at a networking event, no matter how much I enjoy talking to someone, I am also working. It's easy to chat all night with someone I already know or someone that I'm hitting it off with, but hopefully we're both there to increase our exposure to new opportunities and people, so I try to reference that when I break off a conversation.