Being that I was born in the mid 80's, I grew up with access to computers. From the time I was about 7, I was on a computer pretty regularly. Now, I'm on the computer for the majority of my day at work and then at home. I remember getting the "Free 1000 Hours of AOL" CDs in the mail. I was there as Instant Messenger and Chat Rooms became popular and then died out again. I remember seeing Smileys for the first time and going crazy with them. Personally, I don't find LOL, FWIW, IIRC childish. Especially with the advent and skyrocketing popularity of text messaging, these things are great. My wife can text me, "I'll be home at 7." and I can respond, "K" and be done with it. I don't have to answer with, "Okay dear. See you then!" Or if my cousin sends me a funny message, rather than typing out, "Dude! That was hilarious!" I can just simply text, "LOL!"
Think of it as digital shorthand.
Should it be used professionally? Never. Should it be used in serious conversation (email, letters, etc.)? Absolutely not. In casual talk on fora and other e-outlets, I think it is perfectly acceptable and is the norm.
That's just my 2 cents... But what do I know? I'm just a young whipper snapper!