I used to use MSN, Yahoo, and a number of different accounts with Pidgin as my way of talking to people in a post Bolt world, The Problem was that Bolt died a death; replaced by the shocking My Space, and then by the at the less shocking Facebook.
With Facebug came a while later Facebook Messenger, a rather shocking Jabber chat system, that while shocking has managed to fill my main requirement for a chat system; that being that it is Unified, no more having to keep SMS on your Phone, IRC (yes I still use IRC) on my Mac, and Yahoo / MSN / AIM etc slowly being replaced by well Facebook 
On My Phone these days I use WhatsApp to talk to people in other countries, It can support group chats of upto 30 members, and has multimedia transfer. As such it does everything Yahoo/MSN/AOL ect used to do, but is not as crap as Arsebook (yes if I could get of FriendFace I would).
So why a desktop client?
FaceArseBugBook has unified the IM sphere (just make sure when using it via a Jabber Client _not_ to have the site open)
Having my WhatsApp chats able to be continued from the full size of my macbook pro, close that down and keep going on my iPhone, would make it a very good contender for allowing my facebug profile to die 
But I hear you cry, what about iMessage? iMessage would be great, it allows me to attach a number of email addresses, together with my phone number, and then link my iPhone, (iPod and iPad if I had them) and my Mac together, (a web client may be nice). however I can only talk to people with Macs, no Android, Windows ¿Smart Phone?, Blackberrys, Nokia’s only iPhones.
Which is where WhatsApp wins.
So Come on WhatsApp, let me kill off facebook, pretty please?
WhatsApp may have lost the race…
http://www.viber.com/products/mac/