I have the following:
acidplanet - account dating back to 2003 … it is essentially abandoned 
myspace - dating to about 2005 … again essentially abandoned but it has been updated by myspace as a music page and it looks pretty slick 
soundclick - current, it is the free account and the one I am using now 
soundcloud - I have an account so I can share but I don’t have anything posted 
youtube – I just opened up a new account a couple of months ago and have one thing posted that I edited in Sony’s “Movie Studio”
 
Outside of forums geared toward musicians, it looks to me like youtube is the most broadly used medium for sharing music.  At least in my small world, it is the way people post music to facebook and whenever I am asked to gig or sub for somebody … it is the youtube links that get circulated.
 
Any thoughts?

The Coffee House Band 
(this Juan is for paulo since he is forever looking for it)

SoundCloud has been it since a certain unnamed co-conspirator signed me up a long time ago.  YouTube is probably a good place to go also, but I always feel like I need to do a clever video to with any music.  That's just me, though. 

Yea, a certain unnamed individual had me sign up for soundcloud too.
 
... and I feel the same way about youtube.  I posted a Christmas tune that I did in 8.5.3 but, I did all the editing in the Sony software (just added a colored background and scrolling lyrics)

My first option would be YouTube.
 
I was an early adopter of MySpace and I've tried a bunch of different things, but the traffic analysis demonstrated one constant - no content gets you half as many hits as image. Even if you're an ordinary guy. 
 
It doesn't matter if you don't really have any video idea. The best self-promotion tool will always be a cute kitten video - you only need to use your music for the soundtrack and you have a potential hit on your hands. ;)

This one is less well-known but pretty good:
http://indiemusicworks.com/
 
There's also ReverbNation:
http://www.reverbnation.com/

I use youtube and soundcloud.  I started out using soundclic and myspace.  I have abandoned both of those mediums because they were too restrictive and too much garbage being pushed in the listener's face including ads and "music" of others (and I say "music" very lightly there!)

I think the selection of a music host can be a bit cult-ish.  Many years ago I frequented a song writing forum and the host of preference was acidplanet.  Acidplanet is owned by Sony.
 
When I started here everybody was using soundclick.  The free soundclick player is fine when posting something in the Song Forum (you click a link and the little blue play player comes up) but if you post a link on a facebook page the image of a rap artist or metal artist (or some such thing) may be shown.  I’d like to be able to post on non-musician forums or other various forms of social media and not advertise somebody else’s music.
 
Many years ago I wrote a song called “Sarah, A Witch’s Fate” about the Salem witch trials with a Johnny Cash / Stevie Nicks sort of vibe.  There is a new cable series out called "Salem" and I wanted to post the song on a facebook page.  When I inserted the link, a picture of a rap artist was shown for the song.  Nothing against the rap artist, it is just not the image I wanted to present so I did not post it.
 
The myspace page and player looks really slick but I have yet to see anybody use it and I am not sure how it would appear when posting on a “timeline”.

These days I have my own website, but still have a couple of old Soundclick pages that I can't bring myself to delete. Had a MySpace until they changed it a few years back, then it got abandoned.
 
Never been on Soundcloud, don't like the look of it. YouTube will be the next avenue I think, once I have my project finished later this year.
 
I think I still have some tunes at Motagator too, but haven't been there in years.

Double post.