Some things that have been bugging me/confusing me.

What is a Highest Skill?

Is there no way to play custom maps, i.e. zombie, online?

more to come as I think of them.

You can play custom maps/gametypes with friends online by using the custom games lobby. However, I think the only people who can play are people who are on your friends list or who you invite. You cant use the custom types in matchmaking.

I played Infection last night with some friends online, it was a blast.

i have a question...

This game cant be that good can it?

Some love it some dont, some think the single player game is all it is and not try the online co op, find the skulls, play the multiplayer or make their own madcap maps
Single player is not all that but when you add the other features it is

I really like the multiplayer personally.