Acronyms like lol, roflmao, fyi, fwit, imo, imho, etc were in use long before poker chat rooms and have found instant acceptance and extensive usage in 'texting'. All the abbreviations currently used were also used before the days of websites when early computer users (like me) were only able to get 'online' thru use of message boards and bulletin boards.
Early systems were also very expensive to log into and were 'dial-up' based, so time was critical and abbreviations helped keep typing time to a minimum.
'imo' may be overused, or is just that there are more people online daily? There are growing numbers of computer users daily and seldom do we see people giving up the internet or their computers. The difference, as I see it (or.... imo ;-) is that it is a qualifying statement. As Zach noted, there are questions that could have multiple answers and the 'imo' just clarifies that the response from that person is just 'one opinion', not a locked in unquestionable reply.