As pointed out by @Sven Yargs , "Up to much?" is most likely to a truncated version of the question "Have you been up to much?" or simply, "Have you been doing much?"
And by mentioning that this is on Facebook and it is asked towards you, I can say that this question is most likely directed towards you.
How do I respond to it?
Something like "Yeah, [mention something that you've done that is notable (consider how long it's been since you last talked)]..."
For example, "Sure have. I went skydiving yesterday - awesome! What about yourself?"
Green's Dictionary of Slang has in whack (with) as taking equal shares, and also whack as a share or portion. I think that is what is meant here; "you'll get your share".
Among the numerous other meanings of whack is also a prison sentence, and the phrase cop your whack means "to get your just deserts". Perhaps this sentence is a play on the multiple meanings; "you'll get your share of the good things - and the bad".
You're right in that you wouldn't find slang in official titles. However, according to Wiktionary, that definition is not slang, but is instead an archaic definition:
(archaic, sometimes used in titles) To a great extent or degree.
Members of the Queen's Privy Council are styled The Right Honourable for life.
In other words, it's a synonym for very. Those who are Right Honourable are honourable to a great degree, more than just The Honourable, but not quite so much as The Most Honourable.
Best Answer
As pointed out by @Sven Yargs , "Up to much?" is most likely to a truncated version of the question "Have you been up to much?" or simply, "Have you been doing much?"
And by mentioning that this is on Facebook and it is asked towards you, I can say that this question is most likely directed towards you.
Something like "Yeah, [mention something that you've done that is notable (consider how long it's been since you last talked)]..."
For example, "Sure have. I went skydiving yesterday - awesome! What about yourself?"