I've been reading a lot about this topic lately. One thing that seems human nature, is that everyone DOES make an instant first impression, whether they like to admit it or not. It's sort of instinctual to do that, hard-wired into your brain function. Not saying it is accurate, just instant.
When someone makes a *bad* first impression on you, you tend to avoid that person from then on, and so you never get a chance to collect more information and change your opinion.
Similarly, when someone makes a *good* first impression, it takes overwhelming evidence to the contrary to change your mind, which requires being around them for a long time.
The one glaring instance in my case of a wrong first impression was my former boss. When she first came on the job I thought she was great. I was almost the *last* person in the company to change my mind. If I had not been forced to interact with her day-to-day, I would have never changed. This woman was truly evil, but she had a way of making a good first impression. There was about a 6 month break in period for new hires to figure her out. Outsiders never did.
I've generally never hung out with large groups of people before, just one, maybe two, good friends, so this is new to me. Why I find it so fascinating. (Well I find most things fascinating.

Ha!)