I am absolutely dreadful with one-on-one conversations with people who I don't know very well. However, if it's with family or friends I am comfortable around, then I am fine.
I find myself much more at ease in groups, because the conversation is always flowing, and I can just interject when I feel comfortable or if I have something of value to say. Also, since everyone is more focused on the socializing, they don't really notice if I'm quiet and just staring at the floor instead of joining in.
If I'm only with one other person, my mind just goes blank because I feel too much pressure to keep the conversation going. I either just give generic one word responses, or I say some truly random things that can only leave the other person thinking that I am utterly insane.