... excepting AU, of course ...
Plurk recognises and accepts SL avatars. The others either ignore it, or, in the case of FB, actively try to stamp out avatar accounts.
hmmmm... That's almost enough to make me think about adding Plurk to my arsenal. I've only seen it used on a 'smart phone', and I don't have one, but I guess it can be a desktop thing, too.
I use flickr most often from all those.
It's Plurk. For the simple reason it creates some kind of online offline connection, at least for me. It makes a great support platform too. Almost like realtime, like IM, but not. You're online and you're not. And it still connects you to tons of other likeminded people if you wish. Private or public. Neither Twitter or Facebook could do something similar.
Plurk wins out because so far it is closest to realtime chat amongst the choices given here. It is more socially engaging whereas Twitter and Facebook are like posting on a billboard without really caring that anyone interacts with any statement that you have made. I feel that AU will embrace more of the Plurk interaction than any of the others and that will help its success along greatly. This is hinted at by Linden Lab's addition of Plurk prominently within its share button on XStreet rather than buried deep within the "More" option. With Facebook becoming more and more unfriendly towards Second Life residents (and who really cares because Facebook is shit anyway) it would probably behoove Xstreet to replace the instantaneous Facebook icon with the Plurk icon.
Twitter is useful only on the broader plane of virtual worlds in general and not very friendly at all when it comes to Second Life commerce or events. Twitter is for speculating about virtual worlds, not engaging them.
i'm fairly dumbfounded that Plurk is so huge amongst us SLers. But hey, whatever floats our boat.