The truth is, we are all affected by the people we interact with, many of whom we don't even know personally
we don't know what? those peoples or this truth ?
The statement refers to the people we interact with. Don't be misled by the comma after "The truth is" ... think of it as a substitute for the word "that" in this case.
This construction is seen in similar statements, like
It's kind of a loophole in what you would think of as normal usage for the comma, but it is in wide circulation. The fact of the matter is, pretty much everyone uses it that way.