You certainly can post to the web with adobeRGB, you just have to understand that if someone views your image that doesn't have a monitor capable of displaying the adobeRGB color space, which is probably 95% of viewers right now, and if they are using a non color managed browser, such as IE, which is like 75% of the web, then your colors are going to appear less saturated then they should. That's why it's best to just stick with sRGB.
Of course the downside of sRGB is if someone views your image with a wide gamut monitor, and is not using a color managed browser, the images will appear too saturated. Thus my comment that there are simply a lot of variables you can't account for.
Last time I checked, facebook as well as several other social sites I use, are stripping profiles as well as compressing images by around 30-50%.
This is easy to test: Note the filesize of your image and use your favorite utility (I prefer exiftool) to view the metadata in an image. Upload it to a site. Download the fullsize view from the site. Note the d/l image is significantly smaller, and the tags (including colorspace profile) have been stripped.
Yeah you will drive yourself crazy trying to control all the variables, which is why I suggested just saving in sRGB and hoping for the best. It probably works best in most situations.
I know this is slightly off topic from the original question but does capture some of the other comments in the thread. A while ago I was trying to understand some of this, and I found this site/test interesting:
