As users connect and disconnect from the network, all the pc's that they were connected to can become temporarily invisible, and unseen to your particular connection. If you're referring to the Limewire counter, it's very bugged (and Limewire freely admitted as much, and they removed the counter in v1.9, replacing it with an ad banner )

Typically, your client will keep a partial list of past connections, so that if a nearby node logs off, you can still (hopefully) reconnect to that portion of the network that has gone silent.