mmm... when I worked at my last job, we used to have parties(non-holiday) on company time. it was great, drink some margaritas, eat some chicken wings, bring a beer back to your cubicle, then leave (after allowing 1 hour for each drink consumed of course).
But at my new job, they have parties after work on Thursdays. It makes it a lot easier to tell them that I honestly have something to do Thursday nights and I can't go out.
As far as going out for lunch.. I don't see a problem with that, especially at first, people are trying to get to know you and all. I'm not an extremely outgoing person, so maybe its just easier for me to limit my association. I take most of my lunches by myself now.
whenever there's a company lunch , i go... not all the time. but i do. just go our for lunch... have a free meal. come back to work. i see no problem in that. but usually i go home during lunch and take a nap.