Yes you count how many views, likes, re-posts, comments, shares, and additions to collections, but the date of a user's membership to Hello Poetry is also a factor.
If a user is a new member, it is much easier for his/her poems to trend. It is a cool thing because trending acts as a form of encouragement.
However if you've been a member for a while, having your poem trend is harder. Which is a cool thing as well because you already leveled-up and its now a challenge!
At least, that's how I think it works.
Based on F.A.Q. How does a poem trend? Whether a poem trends is based primarily on measurable activity like reads, likes, and reposts, though there are a number of other smaller factors that in aggregate can greatly influence whether or not a poem trends.