I was going to write a detailed introduction to IRC but this has been done to death. Instead I will list the reasons I think you should use IRC. You can find a very comprehensive introduction on how to set up your own irssi IRC client at the following location http://quadpoint.org/articles/irssi/
There is of course many more IRC clients to choose from but having tried a few, I stuck with a combo of irssi and screen I also found that is possibly the most straightforward combination to use.
I can start off by describing what it isn't.
I've been an avid IRC enthusiast for many a year. In my opinion, It's the superior way to chat and share information online. It takes full advantage of the concept of channels; allowing you to surround yourself by people interested in the same topics as you and as a result, encourages the growth of online communities. The twitter #tag is likely to have spawned from these ideas.
If you are technically inclined or enjoy flowing discussion, you may find it a pain to have open conversation on the likes of facebook, you are probably friends with too many stupid, selfie sharing, drama merchants to enjoy the experience fully. It's also mostly unlikely that you would meet new people on facebook (Stalking isn't the same as meeting people.). Other places where you could meet new people such as twitter, are limited to 140 characters. much twitter, many conversation. IRC connects people better than any of these alternatives. It's simple and reliable, there is also the added benefit that 99% of the time, nobody is trying to sell you anything like on social media.
If you are technically able, you can script IRC bots to do very useful things. The simplest example that I use regularly is putting RSS feeds into a channel. This could be updates from your favourite blogs, new houses for rent in your area, vulnerability disclosures, new torrent releases, server monitoring information, stock market prices... BASICALLY ANYTHING. It's nice having such things in a centralised place.
It is also possible to get more involved in communities. freednode.net is full of interesting channels and people. If there is an excellent online community, it is also very likely you will find it has an equally great IRC community. As many of my blog readers probably know, I participate in bug bounty programs for example, what better place to meet people of similar interest to yourself than on IRC (e.g. IRC at irc.freenode.com in the #bugcrowd channel). Not only can you contribute and share findings, you can learn of new bug bounty programs, new vulnerabilities or even get instant feedback on questions you may have. Another major benefit is the potential of learning from others within the community and making friends/associates.
This idea of course extends to all areas of interest. You listen to music? Why not meet others who also do and expand your collection by referral. IRC is people driven, free and provides chat centered, instant communication. I feel more people should return to it, before we all end up being 140 character speaking, narcissist, socially awkward creeps.