The reason this works is because you can put double-brackets around certain 'magic' FB numbers - users and pages, for example.

Check out http://www.facebook.com/pages/Poker-Face/129627277060203 and note the number.

So if you make a page or profile, you can use the page'd or user's ID in double-brackets and it will show a tiny thumbnail of the pic.

edit: There's a "love.org" page that works as [[love.org]] - a nice heart. So now I'm sending my wife [[my_id]] [[love.org]] [[her_id]] - thanks to this thread. :)

EDIT: Another redditor (some of my comments are out of order) wrote a better translator - aescnt's comment and their translator with preview

EDIT: A redditor wrote a generator that uses a good (if pink-on-white) alphabet:

Preview - Generator webpage - post from d-serious - yay!

EDIT: An alphabet from a post below from user blow__me:

[[107015582669715]] = A
[[116067591741123]] = B
[[115602405121532]] = C
[[112542438763744]] = D
[[115430438474268]] = E
[[109225112442557]] = F
[[111532845537326]] = G
[[111356865552629]] = H
[[109294689102123]] = I
[[115636698451811]] = J (hard to read, but legit)
[[116651741681944]] = K
[[115807951764667]] = L
[[106596672714242]] = M
[[108634132504932]] = N
[[116564658357124]] = O
[[111669128857397]] = P
[[107061805996548]] = Q
[[106699962703083]] = R
[[115927268419031]] = S
[[112669162092780]] = T
[[108983579135532]] = U
[[107023745999320]] = V
[[106678406038354]] = W
[[116740548336581]] = X
[[112416755444217]] = Y
[[165724910215]] = Z (dark on white, different from other letters)

Preview of above: http://i.imgur.com/LgPEM.png -- looks good except the original Z was a troll... Z code above is good, from JoeyJoeC

M E R R Y
C H R I S T M A S

[[106596672714242]] [[115430438474268]] [[106699962703083]] [[106699962703083]] [[112416755444217]] [[115602405121532]] [[111356865552629]] [[106699962703083]] [[109294689102123]] [[115927268419031]] [[112669162092780]] [[106596672714242]] [[107015582669715]] [[115927268419031]]

(To test, send in a message to yourself on FB)