The biggest challenge they face with subscriptions, as mentioned by other posters, is that the majority of features that users want are available in the free tier. Most of the subscribers I know (myself included) pay the subscription fee because they want to see the platform survive, not because the subscription tier is dramatically better than free. This model works for non-profits (kind of), but probably doesn't work for a company sitting on $42M in venture funding.
Therefore, they quickly need to a) make the subscription tier much better than free to pull free users over and/or b) pull free tier features behind the paywall. Both are very challenging. If 'a' was easy, you'd think they would have done it years ago. And 'b' is going to risk user churn, which will ultimately erode their network effect. This makes me wonder why they're not considering advertising to free-tier users? Haven't most people accepted the "ad-free for paying users" model?