Subbly has a Facebook Pixel integration, but I don't feel it is complete. It tracks page views, but conversions still require us to deal with cumbersome javascript.
Also, Subbly has one conversion page which rules both subscriptions and one-time purchases. This doesn't make sense. Subscriptions should be tagged as "Subscribe" events and One-Time could be tagged as "Purchase" events (or also "Subscribe" if user wants to).
In any case, this was a process that I just went through. It was a lot of trouble to test and validate. I think Subbly can make this better by asking us for:
pixel id
conversion tag (subscribe vs purchase) for one-times
conversion ID for each product
And it can auto-label all subscriptions as "Subscribe"
And in all cases, it can automatically pass the currency and value.