Sounds to me like they likely did something stupid without realizing it, e.g. getting PTC traffic. Or having sites without clear privacy policy, or sites with offensive materials. (e.g. if it is a forum and someone posted content that is of adult nature, that's a problem, and you may not even know about it until it's too late) And most important of all... I think what Google does is look at their conversion rates... if your advertisers are not making profits from your sites, you are USELESS to the network! Is your traffic targeted? Does it actually APPEAL to your visitors? Do your visitors purchase things online, or are all of them 13 year old kids that like to click?
Does your friends know each other website url? Maybe all the clicks were concentrated in one geographic location?
They probably told their friends they had a blog or a site. I never tell anyone I know except for very rare cases about my blog or site due to this factor alone. People think they are helping by clicking on advertisements, but it hurts adword users and yourself mostly.
I think they do not ban adsense account but a site. You may contact them via this form and their online google forum http://www.google.com/adsense/support/bin/request.py?hl=en&contact_type=appeal_form&rd=1 http://www.google.com/adsense/support/bin/request.py
in past 30 days I have seen at least 14 sites violating adsense policies in ridiculous ways. Simply had to complain once. All 14 are now banned.
those days Google not only ban adsense account. They also ban Adwords accounts with no hesitation. Nobody knows why....
Google's adsense division is broke? AHAHA For every 7cents they pay a publisher they get 40 to $1. How on earth can you go broke with that return! Not True. I know why actually. Its because they are breaking the rules!
I've been banned before because of invalid clicks but I've learned my lesson and never click on it ever again. Never got ban so far - already making $700.00.
Agreed that Adsense isn't broke. But in fact they pay out slightly more to a Publisher than what they keep themselves.
This is completely true. A big reason people overlook is conversion rates. If the traffic is targeted and actually interested in the ads you have on your sites, you will have no trouble with Google because they will make money as well as keeping their advertisers happy.