Since Facebook disabled XMPP (Jabber), the options for connecting to Chat via a third party plugin are limited. Miranda NG, my favourite IM client, has a Facebook protocol which I was using. I had been getting captchas for sending or posting totally innocent links, like even Wikipedia or Slashdot, and even via the web interface. I didn't know what was causing this. Then I switched from the stable to the development Miranda NG version so I could use the SkypeWeb plugin. Soon after that, Miranda NG said my computer is infected and needs to be cleaned. After doing a bit of searching, I found that others had similar problems due to Miranda NG. Since then I intermittently can't send links at all.
It has been a few days since I stopped using the Miranda NG's Facebook protocol, the problem hasn't gotten better, and I'm annoyed. I don't want to use an instant messaging application which has the capability of blocking messages based on content. Much older IM protocols which allow direct connections between clients are so much better. I'm also not happy with their ability to block links in posts.
Basically, Facebook is a piece of shit which I use because others use it. Social networking should function via an Internet standard, not via proprietary web sites. It should be distributed, so you can select or run a server, instead of just logging in to one place.
I'm deactivating Facebook because it's too annoying now. Maybe eventually when I reactivate, it won''t annoy me with captchas and link blocking.
The purple-facebook plugin for Pidgin claims to use the protocol used by the Android Facebook Chat application. That could work better and not trigger captchas. Pidgin is worse than Miranda, lacking in features and flexibility. It's okay though, and I may start using in in Windows because of this.