Does it matter what is classed as viral? If something outperforms your normal readership, I would consider that to be viral'ish. I had a viral 'for me' article that has so far hit 29k views and so far has earnt me over $1,600. Maybe it's not viral by other peoples standards, but it's certainly good enough for me.

Now the fun part comes when you try and re-create that success. Though I try not to put too much pressure on that, and just create articles that I enjoy writing and that are of value to my audience.