Hi Adviso,
I've dealt with a few massive URL changes over the past year and reluctantly had to let the social shares go and just start over again. When you are dealing with thousands of pages, it simply is not going to be worth the time and effort needed to transfer them all over.
Ultimately, they are just a number that is displayed on your page and have yet to prove any significant value. If the content is good and still relevant, it will be shared again. Social Proof is nice, but good content doesn't need it. People will recognize it on their own.
This is the best article I have found on how to do it for each social network:
http://searchenginewatch.com/article/2172926/How-to-Maintain-Social-Shares-After-a-Site-Migration
Good luck on finding a solution that works for you.