Exactly. Facebook is only useful for keeping in contact with your friends as long as the connectivity graph of your friends actually only includes real friends.
If a friend posts a status update, and a friend of that friend comments, you see the comment. The likelihood you will see comments from some random person you will never meet increases by the sum of fake friends of all of your friends.
Even if you have say, 100 real friends, but each of them has ~5 fake friends, you are exposed to comments from 500 random strangers.
You may be right, but I've never seen it happen. I have over 600 "friends", many of which I've conversed with online, but none of whom have commented, posted, or tagged me on -anything-. The exception being the people whom I've met in person.
If a friend posts a status update, and a friend of that friend comments, you see the comment. The likelihood you will see comments from some random person you will never meet increases by the sum of fake friends of all of your friends.
Even if you have say, 100 real friends, but each of them has ~5 fake friends, you are exposed to comments from 500 random strangers.