I found this as a Facebook note I wrote four years ago. It's still relevant.

Dated February 13, 2011:

Here is a short summary analysis of the ways in which the millennials express mirth in text messages and on miscellaneous social networking sites.

Lol: A common man's version of mirth. Less sophisticated (even soccer moms use this), but it gets the point of across.

Ha: Generally, when this expression is employed, the preceding statement was not found humorous by the sayer of this monosyllable.  It is the equivalent of making an insult that shoves someone over the balcony of the eighteenth floor, then throwing a flat pillow out on the ground for the person to land on.

Hah: A hybrid of "Ha" and "Haha."  Possibly the result of the sender's speed at typing, or else an intentional subtraction of a consonant. Nonetheless, this is a sweetened version of "Ha."  The sender has more compassion than a sender of "Ha" and/or might actually find the preceding statement funny.

Haha: The classic expression of mirth.  Hard to go wrong here unless the recipient of the "Haha" was expecting more gusto in the sender's response. (Never send solo unless you're that person.)

Hahaha: "Haha" on the next level.  Whatever was said preceding the "Hahaha" at the very least elicited a smile in the sender of the "Hahaha" and possibly even a cackle.

HAHA: A succinct version of "Hahaha" if not more emphatic.

Hahahaha, Hahahah, Hahahahhaaahhh, HAHAHAHAH, etc.: You get the picture.  Whatever was said before was beyond "Hahaha."  It was uproarious.

Lmao: A layman's expression of amusement.  This term would be more acceptable if its acronym didn't include a vulgar term for a donkey.

Lmfao: An extended version of "Lmao" including an even more vulgar reference.  However, the term may be effective given context and the relationship between the sender and recipient.

Teehee: Implies childish amusement at whatever was said.

Hehe: Just don't.

Rofl: If "Lmao" was the seventh level of hell, this is significantly better.  "Rofl" retains the ordinary man's signature, yet subtracts the vulgarity.  Cousins of "Rofl" include "Rolfwaffle," "Roflcopter," and "wafflecopter." (Note: This expression is not as well recognized as others.)

Omg: Enough said.

Baha and its cousins ("Bahaha," "BAHAHA," etc.): Similar to the "Haha" family, but with extra hilarity accented.

Punctuation: This can be used to emphasize any of the terms mentioned above.  A period brings finality to the laugh; there might not be much to add concerning the aforementioned statement. An exclamation point extends the emphasis on the hilarity; this is a compliment to the receiver of the "!" A question mark denotes doubt in whether the sender of the previous statement intended the statement to be funny and/or expresses derision/non-amusement at the preluding statement.

Lollllllll: This is an alternate form of underlining the humor found in the prelude statement.