Note: I updated this post to add a third tool, Twitodon which was suggested to me after I published the original post which only mentioned Debirdify and Fedifinder.
I have had a few people asking me lately if there is a way they can find people they follow on Twitter on Mastodon. Here are some tools I have tried that will help you find the people you follow on Twitter, check to see if they have a Mastodon account listed in their Twitter bio, and then provide you with a csv file that you can then import to Mastodon so that you can follow them in Mastodon.
All work in a similar way as they look through the people you follow on Twitter to see if they have what looks like a Mastodon account in their Twitter bio, which is a good prompt for you to add your Mastodon handle somewhere in your Twitter bio for others to find you if they use either of these tools.
All also need permission to log in or authorize the service with Twitter so your comfort level mileage may vary. Biridfy does give you the option to immediately log out once you complete the search and download the csv, so that is a nice feature that might make you feel more comfortable with authorizing unknown apps. But I don’t have any reason to believe that these services are not sincere efforts from developers to help others bring parts of their network to Mastodon from Twitter.
Once you run the service & download the csv file of Mastodon handles it finds, you can then log into your Mastodon account and upload the file using the Mastodon import feature which you will find in Preferences > Import & Export > Import. Make sure you set the toggle to “Merge” so you don’t inadvertently delete the people you already follow on Mastodon.