I'm not quite sure what your question is. Just enqueue one request
for each user; BatchFB will take care of fitting them into optimal
batches.
You might also be able to double-up the amount of work per request
using field expansion:
https://developers.facebook.com/docs/reference/api/field_expansion/
I'm not sure about this, but you could try doing the equivalent of:
?ids=u1,u2,u3&fields=posts
...and if that works, you can do both this and use batching. You'll
have to do some experimentation to find out what the optimal
combination is.
Jeff