While most people use this technique to protect their email addresses from spam bots, Facebook converts addresses into images for a completely different reason. They want to prevent their own users from exporting email addresses of their own friends out of the Facebook wall (through scrapping).
Use Facebook-generated Email Images outside Facebook
If you right-click and copy the web address of any email image inside Facebook, the URL would be something like this:
where ABCD1234 is a unique string that is different for every email address and 8.7 is the font size that is used while rendering the image.
What’s interesting here is Facebook doesn’t require authentication for these URLs.
That means you can directly mention these image addresses in your Twitter messages, in online forums or any other public web pages where you are worried about mentioning your email address in plain text as spam bots might harvest them for the purpose of spamming you.
So the next time someone wants my email address on Twitter, I can point him to this Facebook URL – http://bit.ly/my-email. If I want to embed this email address images on a web page, the code would be:
<img src="http://www.facebook.com/string_image.php?ct=ABCD&fp=20" />
You can vary the font sizes by changing the value of the fp parameter in the URL as I have done in the following examples – these are all dynamically generated images.