Open a specifc URL in Fluid.app
I had the same problem and wrote a little AppleScript to handle this. You can find the app here: https://github.com/biafra23/Default-Fluids

You need to make this script your default browser. 
This can be achieved with Safari: 
Menu -> Safari -> Preferences -> General -> Default web browser

The source is in https://github.com/biafra23/Default-Fluids/blob/master/default_browser.app/Contents/Resources/Scripts/main.scpt 

It contains sections like this: 

if this_URL contains "facebook.com" then
  tell application "/Applications/Fluids/facebook.app"
    activate
    open location this_URL
  end tell
else if 
...
else
  -- default browser here
  tell application "/Applications/Firefox.app"
  activate
    open location this_URL
  end tell
end if


You need to make changes to those according to your setup with the AppleScript Editor. 

It is not as convenient as choosyosx but its free.