It has some problems (It hampered me, for instance, by showing me an announcement of a locked entry, an icon post that showed only the numbers and one with a bad html tag so the selection was mostly full of gibberish.) In spite of that, here's how I did:
you have the option to guess 10, 20 or your complete list, and it gives you 4 possible choices. Since my flist is so big, I went with 20.
Guess That Friend
You have a total of 307 friends
You requested to guess 20 entries.
None was skipped due to invalid content (surprising?).
You made 20 guesses.
You got 19 correct.
Doing the math for you, that means you got 95% correct
So now you know - I generally do read my flist, and apparently I can pick many of you out by a turn of phrase, or subject matter. Also - post more stories - that Author line is a giveaway (Of course I would get it on a drabble I had actually read and could place.)