Whatever you want to link to should first work if you were to just copy-paste it into your browser window. If it won't pass that test then there's something wrong. I can't get your link to work, because:
your app id is "MYID" - you need a proper Facebook app id number here.
caption= Check out my MyGame score of $"& HighScore &
&link=http://flappy2bird.blogspot.fr/
- there's a hard return and two &'s next to each other when this is decoded.
and decoding the last part:
redirect_uri=http://www.facebook.com/&name=MyGame&description=Can you beat my score?"
Is your game registered on Facebook as MyGame? If not, that might cause a problem...