1) Export all your Facebook friends’ Email address :
“Go to address.yahoo.com and click the Facebook icon. A login dialog should pop-up, just sign-in with your Facebook credentials and within seconds, you entire Facebook address book will be available inside your Yahoo Mail Account. Once the import is done, click this link to download a CSV file with the email addresses of all your Facebook contacts to your desktop”. [via. labnol]
2) Now open the CSV file you downloaded using MS-Excel or any other spreadsheet program. Then you will find the column with Email-addresses of all your Facebook friends. (The column is probably E in all cases). Copy the whole column.
3)Paste the whole column in Notepad or any text editor. Now you need to add comma (,) after each email address. For this Click on Edit–>Replace. Replace all .com with .com,. Select all and copy.
4)Go to twitter.com and login. Click on Find people at top right. And click Invite by email. OR you can simply go to the page : twitter.com/invitations/invite_by_email
5) Paste the email-addresses there and click Invite. You are done.
Now you will follow all the Facebook friends who have Twitter profile with the same e-mail address. Those who don’t have a twitter account will get an invitation to Twitter on their e-mail address.