Facebook maintains a list of websites and urls that’s spammy, abusive and potentially unsafe for minors. All these urls are blocked from appearing anywhere in Facebook updates, comments, notes and chat.
Usually, whenever you type a url in Facebook, it automatically retrieves the link and shows it as hyperlink.
This a nice feature, but if someone sends weblink of a p0rnographic content, image or any other objectionable content then the hyperlink wouldn’t be visible, rather a pop-up box appears up containing a warning message.
In the message, you be notified that Facebook blocked the entered url and posted it just as a text rather than a hyperlink.
Here is an snapshot of what you’ll see,
Sometimes, wrongly entered urls can also show this message.