One person not believing in the existence of another is relatively new to humans, and our social rules are not set up to handle it.
I think the idea that one human not believing in the existence of another is in some way rude or disrespectful has already been somewhat established, and is often used (mostly implicitly) as reason for believing in God. (ie, a girl I dated once claimed that she imagined herself becoming an atheist, imagined God's subsequent disappointment in her, and this convinced her somehow of the existence of God)
A protocol for encountering an entity you didn't believe in has also been established:
..."This is a child!" Haigha replied eagerly, coming in front of Alice to introduce her, and spreading out both his hands towards her in an Anglo-Saxon attitude. "We only found it to-day. It's as large as life, and twice as natural!"
"I always thought they were fabulous monsters!" said the Unicorn. "Is it alive?"
"It can talk," said Haigha, solemnly.
The Unicorn looked dreamily at Alice, and said "Talk, child."
Alice
Some research says that lurkers make up over 90% of online groups. I suspect that Less Wrong has an even higher percentage of lurkers than other online communities.
Please post a comment in this thread saying "Hi." You can say more if you want, but just posting "Hi" is good for a guaranteed free point of karma.
Also see the introduction thread.