What is up with my Facebook chat?

What exactly causes Facebook to prompt users to fill out a CAPTCHA when sharing links in Facebook chat?

• Sometimes, when users are using Facebook Chat, they'll be prompted to fill out a CAPTCHA before they send a message that includes a link. What exactly causes this? Is it simply domains that have been reported frequently?

• Answer:

  Facebook recently released a blog post discussing a high-level view of how they do this. If you're a software engineer, here it is: https://www.facebook.com/notes/facebook-engineering/fighting-spam-with-pure-functions/10151254986618920. Otherwise it won't make much sense. But to summarize, Facebook takes numerous aspects or "data points" into account. It's not just about the URL being shared, its also about the user's behavior (how often they're sharing, is it the same link, are people interacting with it, etc.). One can reasonably assume that a score is calculated and returned that means "OK",  "Warn", or "Block". If the "Warn" gets returned, then they'll ask to verify that you're a human.