The use of 'favorited' as a verb has only really emerged in recent times due to its use on several "social" websites where it is seen as an act of endorsement.
Many dictionaries have yet to recognize the word, despite it first coming into the popular lexicon in 2004 via photo-sharing website Flickr.
To favorite or not to favorite?
I can understand why the use of the verb 'to favorite' or 'favorited' might annoy some grammaticians, I think it's also worth remembering that the English language has always been a dynamic, ever-changing and adaptable beast.
Just look up the etymology of the word 'favorite' in the first place and you'll see what I mean.
I like this quote from Stakcexchange.com that sums up the usage of 'favorited' pretty well:
"Some people dislike the verb as ugly, others say it's "an abomination of a non-word" and "emblematic of cultural necrosis & linguistic stupidity", but it's commonly used by Flickr, YouTube and Twitter (with 75m, 800m and 200m users respectively) so it doesn't look like it's going to disappear any time soon. There's no entry (yet) at the Oxford English Dictionary, so when was this internet usage first used? Who popularised it and when?"