I thought there was something between us, but he won't add me on fb!?

[050]

I met this guy at work recently. He's only visiting for the summer and is leaving in a week or so. We've spent a lot of time together at work though. I took a liking to him right away. We joke around a lot. Sometimes we're sassy toward each other but most of the time we're flirty (for instance he's portugese and I'm Mexican and he calls me "querida" which basically has the same meaning in Spanish and Portugese.)
He added everyone from work to his fb friend's list - except for me. I requested him three days ago and he hasn't responded. He definitely knows it's me because of my name (plus all the common friends we have). If he's not interested in me like I thought, that's fine. Not accepting my friend request makes me feel kind of bad though...I at least thought we could be good acquaintances if nothing else.

Am I overreacting or looking into it too much?
Why do you think he added everyone except me?