Should I reply ey ex-bf's Birthday message on MSN?

We broke up a month ago. I had spent 7months with this guy , we used to have great time together and both of us thought we would marry each other and live happily ever after and spend the rest of our life together.... However we started to have arguments , quite a lot and then 29/9/10 , he broke up with me and he... show more We broke up a month ago.
I had spent 7months with this guy , we used to have great time together and both of us thought we would marry each other and live happily ever after and spend the rest of our life together.... However we started to have arguments , quite a lot and then 29/9/10 , he broke up with me and he said his feelings had gone and he lost all of his confidence about us and wished we could still be friends. But I was kinda trying to tell him I was Happy therefore I even wrote sth on facebook where he could see ( It was about A cute guy asking me out ) . After this , He found a new gf in 10 days since we broke up.
Then I cut off all of the contacts, after one week, his status has been changed to 'single' again so I went to see him and became friends again.
But on the same day, he told his friends he couldn't handle seeing me on facebook and I had been sending my greetings and hope he is getting better in a week. But he still didn't reply.
It's been one and half weeks since I stopped contacting him , then he Left me a message on MSN saying' Happy birthday xxx' (Today is my birthday and he used to say that he would spend this birthday with me)
I thought he would never respond but he did today.
What should I do? Should I ignore it or reply?
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