Dots not Lines
You can think of Prior Art loosely as all of the information that has been made available to the public in any form before a given date that might be relevant to a patent's claims of originality.
The cleanest prior art is citeable and a date can be established for when it was available to the public.
Some examples of citable prior art:
The (electronic) form which gets filled out by Ask Patents looks something like this:
In addition a submission is made by Ask Patents which enumerates elements which are present in a piece of prior art listed in the form without drawing any conclusions.
Another way to think about it is we are submitting dots not lines. The examiner needs to connect the dots himself or herself and form an opinion about whether the submission constitutes prior art for the patent claims in question. Arguments are difficult to get through the AIA pre-grant submission process used by Ask Patents to submit prior art to the USPTO for specific applications.
Hypothetical users doing something on Facebook (even if...of course they do that) starts to look more like an obviousness argument.
It is better for us to find and cite the reference and let the examiner make his or her evaluation.