I believe this originated from the MSN days where users would use emoticons (we call them emojis now) by typing in letters/symbols and it would automatically turn into an emoticon. read more
Shorthand for Yes, Imagine like this: Your parenthesis are a defining principle to smileys; : ) ;) etc, so encase a Y (Stands for Yes) in Parenthesis, defining an emoticon, (Y) and it is a thumbs up because a thumbs up is a way to express approval with your hands, a way of saying yes. read more
To create the thumbs-up symbol on Facebook, click the area where you want the symbol. Enter the shortcut on your keyboard . Type the following in your keyboard: open parenthesis, the letter "Y" in caps, and then closed parenthesis. The end result should look like this: (Y). Click Enter. Once you have entered the shortcut, hit Enter on your keyboard. read more