The download button opens the iTunes App Store, where you
may continue the download process. You must have iTunes installed with an
active iTunes account in order to download and install the software. This
download may not be available in some countries.
From Square Brackets:
inCommon allows you to find common friends and followers on Twitter.Simply enter two Twitter usernames and inCommon will match them to find people who follow both of them, and people who they both follow.When you've found matches, you can follow/unfollow them, find matches for them, and open them in your favourite Twitter app (Twitter, Echofon, Tweetbot and Twitteriffic are currently supported).** inCommon does not allow you to Tweet - use your current Twitter app for standard Twitter functionality. **** Twitter imposes limits to the amount of data inCommon can process. You may be told to wait a short while before finding more matches. Sadly there's nothing we can do about this. **** Due to the above data restrictions, inCommon can only process the most recent 10,000 people a user has followed and the most recent 10,000 people who have followed a user. **