As a lawyer, I can say that the addition of a clause banning "fraudulent" or "deceptive" advertising is not a simple matter to enforce. Yes, you can include the clause, but the enforcement hinges on the definition of deceptive and fraudulent. These are notoriously difficult to define with precision and will be interpreted in different ways by different judges and juries. Plus, enforcement will require a lawsuit, the prosecution of which can be costly, while the outcome can be uncertain. Better that Facebook charge a nominal monthly or annual subscription fee and eliminate advertising entirely. Users have to come to terms at some point with the fact that there is no free lunch on the web. Either the user pays or the advertiser does.

Most of these companies (or rather their lawyers) are experts at writing long and elaborate contracts in legalese.
Why not simply put a clause in there that the advertisements must not be fraudulent or deceptive?
That can't be too difficult, can it?