One of the biggest deterrents when it comes to using Google+ has been that you have to start over. Think about it, you've probably spent years cultivating your Facebook account, tagging pictures, adding trite quotes to your info page and most of all, acquiring a bajillion "friends." It'd be depressing to go from that huge, comforting, self-affirming number down to, say, two when you start up a Google+ account, right? Enter Mohamed Mansour's Facebook Friend Exporter for Google Chrome. The extension wasn't made for Google+ migration, but it happens to do the job well. That's probably why Facebook decided to block it.
The extension works by going through and collecting your Facebook friends' email account, which likely serves as their identity elsewhere on the internet (unless they still log into Facebook with an embarrassingly-named Hotmail account from high school, like I certainly don't). In order to keep this from happening, Facebook recently implemented measures that will hide an email account from you if you frequent the owners profile page on a regular basis. While it's troubling that Facebook has taken the initiative to take information that was explicitly shared with you (email addresses between friends) and hide it, taking emails from off the site is technically a violation of Facebook's Terms of Service. Whatever the case, the spirit of this new restriction seems a little bit, well, petty.
For the time being, the extension still sort of works, and a determined user can wiggle their way around the new restrictions. Check out ZDNet for instructions on coaxing the extension to work, but be warned, coaxing probably won't work for long.