Store supporters nationwide rallied in their defense, one supporter even going viral with a Facebook campaign.
More:Hobby Lobby Day: A Fight for Religious Freedom - US - CBN News - Christian News 24-7 - CBN.com
What does "going viral" mean here? And what is grammar rule related to it?
Can I use "become viral" instead of "go viral" here?
And in this case, is "go" used as a linking verb?
I haven't looked at that Urban Dictionary page; I fear that 'going viral' may start to be used in the specific sense of 'using social networks'. That's why I prefer 'become'.
It is a set phrase but it's been used incorrectly in the piece. One supporter can't go viral on the internet. A supporter might post a video on YouTube and then the video might go viral. It simply means something which spreads incredibly quickly on the internet, sometimes quite surprisingly.
The Gangnam Style song/video is a perfect example.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.