Do I understand correctly that Google will try to punish you (or the other party) if you host links in exchange of money but that Google is doing it in its search results ? (And I receive from time to time unsolicited regular paper mail from Google so that I advertise my enterprise through them.)

Page Rank is a neat idea, but making everyone listening to Google so that Page Rank remains meaningful is stupid. What do you do if suddenly Google thinks Twitter or Facebook are to be considered spammy link farms (which would be true) ? Do you ask everyon to delete their tweets linking to you ?