As you can see, Google Feedburner has removed Socialize feature from its left menu on Feedburner account. Webmasters and bloggers hope that it's not a permanent change because this tool is very useful for those who want to link their RSS feed to their Twitter account.
The link has been removed, but hopefully not the feature: you can access to Socialize settings by typing this URL on your toolbar address:
http://feedburner.google.com/fb/a/socialize?id=XXXXXXXXXXXX
Just change “XXXXXXXXXXXXX” by your own ID, that you can retrieve on toolbar address when browsing options of your feed in your Feedburner account.