Deleting Other People’s Comments on Facebook

I don't think G+ (or any other) will delete your content.

I think there is a best practice somewhere that says "do not delete anything" (at least this is how I learned database design), it's just like in nature "nothing is lost, everithing is recycled". :)

They rely on content—think of Google search—sometimes it seems that it knows what you want to search, beacause the more users search, their database grows bigger and bigger, the more database searches that can compare with the more precise is the result. This is true for social media to, they build a profile for every user, and they build the so called "social graph" of the world (read this interview). :)

What they do with the results... first they are showing custom ads, I think they even sale the results to big companies—they have data that is extremly valuable in marketing. For example: a phone company launches 2 new models, m1 and m2, after a few weeks they go to G+, Facebook, and ask them, what is the phone that users talk more m1 or m2, and what phone from a competitor is on top of users talks?

So you need to know that every time you register to a service like this, ALL your data will be stored forever by the company, be it G+, Facebook... you name it.

That's my opinion.

Facebook does not allow you to disallow comments on your Timeline. This article has a quote from a Facebook employee stating:

We think these policy changes support consistency for the Facebook Pages product and encourage an authentic dialogue between people and businesses on Facebook.

Moderation is the only way to accomplish what you seek. An All Facebook article has a better writeup on this (without the quote).