facebook og:image property

i've been using fb social plugin quite satisfactorily but there is one issue which bugs me.
It's that i've specified og:image to be [avatar].
Now, even though the file associated with og:image changes after any update of profile image,facebook continues to show that profile image on its post which was present when the first like was done.
Can you tell me whether this is an issue with FB or CB?
If its an issue with FB,can something be done about it?

Facebook caches the information for a very long time. It'll eventually update or at least it should. At any rate there's nothing you can really do about it as it's stored on FBs side of things.