Be wary of the automated tools on social networks, you might be revealing more than you intended. If you have recently used the ‘Find Friends’ feature that is configured in the official Twitter app for Android & iPhone then you have unknowingly authorised the social network to upload contents of your entire address book to its servers and to retain them for 18 months.
Twitter does specify that it stores certain categories of data on its servers, but is not specific about the retention of users' contacts and address books during the same period for those using the ‘Find Friends’ feature within the apps. The only notice this feature prompts is that it will "scan their contacts for people they already know on Twitter." However, it fails to inform users that their personal contact list will be stored on Twitter servers for the duration of 18 months.
It would be prudent for users to acknowledge the dangers of uploading their contacts online and being vulnerable to third-party threats. You’d be better off following users individually until this feature is tweaked or updated. Another instance where manual effort trumps automation.