RE: Social Engagement Refresh Rate
You are correct that different sources will take different periods of time to acquire.
Twitter public messages, for example, is quite fast.
Private messages, on the other hand, are quite a bit slower.
And YouTube actually states in their API documentation that they are not a real-time system and that comments are available to 3rd parties (like MSE) in a reasonable time (but with no SLA).
Let me check with a PM and get a more specific answer for you.