Humor. Nothing is so effective when used well--or so disastrous when handled badly. Written humor, especially, can land a sale or drive a prospect to your nearest competitor, improve a relationship or invite a lawsuit. No chuckle in the voice or wink of the eye modifies clever words; either the words are funny by themselves or they aren't. Don't try to make strangers laugh this way unless you're sure you know what you're doing.
Even if everyone finds your social messages hilarious, the identical approach may crash-land in business communications. When to joke and when to be serious is a question that differs with every office and industry, but there are two principles no humor-minded business writer should forget.
1. Act your age. No office should be full of always-dour faces, but neither should a business be staffed by thirtysomething class clowns. No one trusts the professionalism of someone who hasn't grown up yet. So to make the right impression, keep written jokes fairly low-key; think chuckles rather than belly laughs. Humorous anecdotes, especially true stories, are best; puns are fine if they don't make people groan so loudly as to stop reading. Knock-knock jokes and "cross x with y" riddles are best left to the kids' books. Things not to do include letting humor overpower the real message and slipping in material irrelevant to the main point.
2. Expect whoever reads your material to be ultrasensitive. If you don't know your intended readers, or if you know them only casually, a safe rule is "never make fun of anyone except yourself." This holds doubly true if you're writing to an individual and are tempted to build rapport by needling him; one person's good-natured teasing is another's cutting cruelty. If someone does complain that your joking hit a raw nerve, apologize without excuses, without defensiveness, and--above all--in a dead serious tone. The worst thing you can do is say something along the lines of "Ohhh, a grumbler; shall I send you some free anti-irritability pills?" This amounts to trying to bully someone into a better sense of humor, an approach useful only for making enemies.
Despite these pitfalls, a good laugh can brighten a dull day at work, which guarantees the writer will be remembered favorably. So if you have a gift for humor, by all means put it in writing!