"Never pay for ads for Twitter and Facebook." and "
Pay for quality products."
Do you mean pay better website to get your products listed?
It's not about quality of products. See the other reply. Without ads, there will be no Facebook or Twitter. The issue with fraud is a very serious and difficult one that creeps into many industries, and ends up sucking the life out of them (hint: check banking or finance!).
FB can still put ads on users timeline pages, like Twitter inserts ads in between the tweets.
But this kind of fraud is a big harm to the social network ecosystem.
Putting ads on the users timeline pages may not decrease fraud. The fraud issue is not where you put the ad at.