I'm not exactly sure what a 'Fanzai' is, but apparently Gareth Barry has a Fanzai account and on that account, he confirmed that Diego Costa indeed did not bit him during their little display of machismo. This confirms Costa's denial, which Chelsea conferred via a spokesperson last night.
"Seen a lot of talk about the incident involving me and Diego Costa in the game last night. For the record, Diego did not bite me."
-Gareth Barry; source: Fanzai via Sky
Costa did receive a yellow card for the altercation, which seems like appropriate punishment. It was his second yellow on the day, which is a bit unfortunate since the first yellow was given because referee Michael Oliver fell for a bit of theatrics from Barry. At least the Everton man didn't try to play up this second incident as a bite, which he easily could have and Diego would now be facing a much lengthier ban than the one-match ban for two yellow cards.
(As far as I can tell, these two yellows do NOT count towards yellow card accumulation for the season as they were essentially converted into the red card and the associated punishment. Costa was on 8 yellows prior to the match, and would have to serve an additional ban if he collects two more yellows before the second Sunday in April.)
Incidentally, this was the very first red card of Costa's Chelsea career, and his first red in over four years (a fairly similar incident, in which the opponent crumpled to the ground instead of standing up to him). Given the way Diego plays on the edge, this is a remarkable feat.
In any case, while there's still conceivably a way for The FA to hand out greater punishment, it looks like nothing beyond the "usual" #CostaCrimes happened here between Costa and Barry. Please remove your pitchforks and give him a couple band aids. Thank you.