How should I react to getting a fan?

thats up to you really..
now that we have a mural i think its okay to say "thanks for being a fan" but i PERSONALLY feel thats a bit forced and meaningless. i see people saying "thx 4 the add" and that takes less than 2 segundos to type and i dont feel it really says anything. its different for people you have interacted with- maybe they left a nice comment on your submission or sent you a prop!? then i think its nice to say something on their wall.
NEVER give props for being a fan..grrrrr it makes props meaningless.

as for fanning them back- some people automatically fã you back, some dont, some just fã in hopes of you adding them back because they feel a high fancount means something. let me tell you it doesnt!
I am INCREDIBLY AWESOME....SUPER MEGA AWESOME and yet i dont have a lot of fãs (or medals for that matter...but still *points at self* AWESOME)

so it is your choice really. personally i will add people who i have interacted with or who like a lot of the same stuff i do.
if they type lyk dis 4 sho i wont add them- no matter how nice they seem. if i see they are a member of only twilight, bieber, aleatório spot and other stuff i dont care for, i will not add them.

you seem sane and are on the etiquette spot AND type well (sheesh you added apostrophes and capitals!!!!! go you!!!) so i would fã you back!

you should also consider that if you like to filter your mural or chat (is the chat still even there? its always turned off) or like looking at scores on quizzes the fãs you see are the people "youre a fã of" so you might want to be choosy :P