Linguist John McWhorter argues in his TED talk that LOL is no longer used in its original meaning “laugh(ing) out loud”, but has come to be a marker of empathy with the hearer (used in contexts that are not particularly humorous). That is, it is used like a discourse markers such as you know or I mean:
http://edition.cnn.com/2013/04/30/opinion/mcwhorter-lol/index.html?hpt=hp_t5