According to the Oxford English Dictionary Online, for the March 2011 release of OED Online, it says it "selected for publication a number of noteworthy initialisms—abbreviations consisting of the initial letters of a name or expression. Some of these—such as OMG [OMG int. (and n.) and adj.]: ‘Oh my God’ (or sometimes ‘gosh’, ‘goodness’, etc.) and LOL [LOL int. and n./2]: ‘laughing out loud’—are strongly associated with the language of electronic communications (email, texting, social networks, blogs, and so on). They join other entries of this sort: IMHO (‘in my humble opinion’) [IMHO at I n./1], TMI (‘too much information’) [TMI at T n.], and BFF (‘best friends forever’) [BFF at B n.], among others. Of course in such a context initialisms are quicker to type than the full forms, and (in the case of text messages, or Twitter, for example) they help to say more in media where there is a limit to a number of characters one may use in a single message. OMG and LOL are found outside of electronic contexts, however; in print, and even in spoken use (see, for example, the 2003 quotation for LOL int.), where there often seems to be a bit more than simple abbreviation going on. The intention is usually to signal an informal, gossipy mode of expression, and perhaps parody the level of unreflective enthusiasm or overstatement that can sometimes appear in online discourse, while at the same time marking oneself as an ‘insider’ au fait with the forms of expression associated with the latest technology.
As such usage indicates, many people would consider these recent coinages, from the last 10 or 20 years, and associate them with a younger generation conversant with all forms of digital communications. As is often the case, OED’s research has revealed some unexpected historical perspectives: our first quotation for OMG is from a personal letter from 1917; the letters LOL had a previous life, starting in 1960, denoting an elderly woman (or ‘little old lady’; see LOL n./1); and the entry for FYI [FYI phr., adj., and n.], for example, shows it originated in the language of memoranda in 1941."