Often, at networking events, people become uncomfortable in knowing how to end one conversation so that they can move into meeting other connections. Consider starting a conversation with an artificial time constraint.
"I need to catch Fred before he leaves, but we haven't had an opportunity to meet yet..." or perhaps...
"I'm on my way out but before I left I wanted to ask you..."
Both options start the conversation in such a way as to imply you have limited time, providing you with a built-in reason to have to cut the conversation short. You have an established 'out'. Smooth, easy, comfortable and natural. Give it a try at your next networking event.