@John
There's a Normalization metric that compares the engagement quality based on machine learning techniques and some statistical analysis.
For example, a good thread is classified based on:
category tags (i.e., what most users are likely to engage)
views vs comments ratio
audience rank (i.e., each user should be pre-awarded a rank based on previous up voting history on that subject tag like in StackExchange community)
question posters rank
Average engagement duration per login session per user per discussion
Aggregated positive upvotes against negative ones
And various other quantization and parameters to fine tune the thread value.
All these require a significant amount of historical data and active user metric collection for comparative analysis against a standardization model.
Such classification scripts won't even take any significant compute resource on their web hosting server.
But such rigorous implementation requires some effort from the developer side.
(I don't think iTalki will be serious enough to implement such mathematical models for sorting their discussion boards like Facebook Discussion forums, StackExchange community, Twitter comment ranking, etc., - all machine learning embedded.)
And Yeah, I know time zone conversions!
Moreover, it depends on the userbase nationality concentration. (Which is think is mostly Asians, since iTalki is originally a Chinese initiative and followed by Latin Americans - projected from Spanish to English learners.)
Also, continent wide time window estimates are kind of a overkill.
Especially, for America the zone mapping can vary from -3 EDT (Atlantic Maritime) to -9 PST (Alaska)
So, 00:00 to 02:00 London can vary from,
21:00 to 23:00 EDT
15:00 to 17:00 PST
(For Overall America alone!)