I made that same mistake, too! I thought when someone friended me, it was mutual, but I have to friend them back.
By the way, you can see the list of everyone who friended you that you didn't friend back by going to your Friend Feed. In the paragraph at top, it says something like, "click here to see or edit your list of mutual friends." That brings up three lists: people you friended who didn't friend you back; people who friended you that you didn't friend back; and your mutual friends.
From there, if you want, you can go to those people's spark pages and add them.