CAPTCHA is an acronym for Completely Automated Public Turning Test to Tell Computers and Humans Apart.
But aside from competition, it may also be that Google blogs tend to be the places where malware seems to congregate. According to ZDNet:
U.S. & World
Facebook has likely started doing this automatically because thousands of spam links are hosted by Google Blogger. Unfortunately, this is now affecting legitimate websites. Meanwhile, thousands of spam links not hosted on Google Blogger are still getting through.
While the problem was reported this weekend, we noticed that we were able to put up links to Google blogs without any CAPTCHA on Monday. ZDNet has since said that it appears to be back to normal, but that Facebook still hasn't commented on the controversy.
It's up to Facebook how it will conduct its business, but when it seems to affect only a competitor -- it may be deemed more than a little coincidental. Perhaps this was just a security tool and not a conspiracy, but only time will tell -- and if Facebook makes links from Pinterest or other competitors harder to post.