First, what you don't do:
1. Never ever say "It was nice meeting you" and walk away. Worst case, you've made an enemy.
2. Do not say "I want to go to the buffet/bathroom". They may want to go along with you.
What to do:
1. State what you want. "I'd like to go meet those people over there."
2. Give them a choice. "Come with me"
wait a second... I do that all the time. what's bad about it?
Really, the best solution is to have people leave when they are no longer interested in talking to me. (I mean, it'd be even better if I could get actual feedback so I'd know if they were bored or if they have other things to do but that sounds like too much to ask.) I mean, I try to leave people places in the conversation where they can leave if they aren't interested in my 'tales from the computer janitor' or whatever, but that's hard to do without sounding like I'm not interested in talking to them, so really it works out best when people are direct.
So yeah, speaking as someone without much by way of social skills, I depend on people leaving, or as you said "ditching" me when they are bored with me- I mean, sure there are subtle signals the socially skilled send so others can tell when someone else is bored. Sometimes I can pick up on those... but sometimes I can't. Like I said, I'm not very socially skilled.
Some people -- not that I would stereotype us computer folks this way -- are extremely averse to social ambiguity, but most people are pretty comfortable with it. If they need to know something (does that chick like me?) they'll find out in the natural course of things (when they ask her out.) In the meantime, if they really, really care, they'll have some anxiety over it, but that doesn't apply to most interactions. You don't really need to know right now if that person found it nice to meet you, and rushing to figure it out only increases the chances that you'll end up believing something false.
Since people are generally okay with ambiguity, it's a no-brainer to them to observe identical social niceties in some situations regardless of their actual feelings. There's no point in alienating someone unnecessarily, especially since you might change your mind later. "Man, what a day. I hated the latte I got this morning, I hated the ham sandwich I got for lunch, I hated work, and then I ran into that wallflower douchebag at the grocery store. Come to think of it, it's probably just me; I hated everything today. wallflower is probably a decent guy. Maybe I'll invite him to my party this weekend." Because things can change moment-to-moment, even especially perceptive people who can see through the niceties treat a lot of what they perceive as provisional. They don't go back to their lair and add a row to their enemies database because somebody was a little frosty at the grocery store.
P.S. The average person's tolerance for social ambiguity is one reason why it's so frustrating to try to get tips from people who have a lot of social confidence. We go to great lengths to figure things out to increase our comfort level and avoid doing the wrong thing; they don't sweat it if something isn't obvious to them, and when they need to know something, they use a low-risk probe that at worst results in a mild but informative "mistake." Usually the real challenge is not an analytical challenge of figuring out the situation but a creative challenge of coming up with something to do or say that will turn out okay no matter what the situation actually is, and will hopefully yield more information so you can refine your approach.
[..] do you like being ditched?
Was it really nice to meet them?
Saying "well, if you'll excuse me I'm going to get another drink/some food/whatever" is fine. They won't follow you unless you invite them (or at least they shouldn't), since that's code for "I'm ditching this conversation, bye!"