I socialize best in small groups (3-4 people total). I do pretty well one-on-one, but I often have trouble keeping the conversation flowing. Strangely, I find that I have this issue more with people I know well than with complete strangers. With strangers, there is so much you don't know about each other that there are tons of topics to discuss. With people you see daily, you've already covered all of those topics, so it can be harder to think of new material.
In smaller groups that isn't as much of an issue since the burden is shared amongst more people, so it makes it easier. The group is also still small enough that you can contribute significantly to the conversation without much effort.
In larger groups, I find that it's just too hard to get a word in. One person will talk, then another will start as soon as they finish. Unless I basically interrupt someone, I find it tough to butt in.