'meself: Are you suggesting that the poster who said "What is breakfast?" was joking?'

Yes.

'Meaning what? That he does have breakfast, but he thought it was funny to say that he didn't have breakfast?'

Yes, probably - although I'm not familiar enough with his eating habits to confirm that statement in its entirety. However, I am quite certain that he did indeed think it was funny to imply that he does not eat breakfast.

'Where's the humour there?'

Some humour can be explained fully - a pun, for instance, can be explicated for the benefit of someone who would not otherwise detect it. By the time the analysis is completed, though, the humorous effect has probably been lost - but at least the person who required that analysis can see how the joke would be funny to those who immediately "get it". Humour of the self-deprecating, ironic, tongue-in-cheek variety, as in the present instance, is far more difficult to explain, if not impossible. I confess it is beyond my ability. However, I would point out the use of hyperbole - the poster implies that he is so far from maintaining a healthy diet, that he doesn't even know the meaning of the word "breakfast", which, given that he clearly has mastered basic literacy in the English language, cannot be true. Furthermore, there is a hint in that hyperbolic question of an alternative, yet perhaps equally valid, approach to life that does not concern itself with the finer points of healthy eating, nor, in all likelihood, with many other matters held to be of grave import by the more somber, sober, grim, earnest, obsessive, and/or neurotic of those among us. Why the expression of such a raffish, cavalier, devil-may-care attitude in response to advice regarding a dietary supplement should strike anyone as funny is, as I say, beyond my limited powers to explain.

Now - how did you like my reply: "Half a six-pack, for some-who-shall-remain-nameless"?