Facebook recently made it so that when someone comments on one of your posts, the friends of that commenter will see that comment, and your post, even if they are not friends with you. And, they can comment on that post of yours, even though they are not friends with you*. Now, commenting on your post seems a rather friendly thing to do, so you may be tempted to send these people a friend request. After all, they came and posted on your timeline. And you have at least one friend (probably more) in common. Sending a friend request seems the neighbourly thing to do with someone who writes on your wall, and with whom you share common friends.