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My Wordpress-based site (dev: volved.dyndns-blog.com) allows users to invite friends via Facebook. When an invitation is sent via an fb.ui send message call, the following is added as the link parameter.

http://volved.dyndns-blog.com/login/?action=register&invite_id={3CBBBEB8-6ACF-4319-AF07-3B6C22C1091D}

The "invite_id" parameter normally causes a redirect. Since finding that Facebook will return a server error 500 when it sees a redirect, I've changed the code to detect 'facebook' in the user agent and not do a redirect. Still however, the fb.ui call is failing. My debugging efforts are hampered because it appears that in some cases, FB does not even test the link. It just fails with code 500. Perhaps this is due to some caching. I haven't figured the pattern yet, but it seems to occur after an unsuccessful ping by FB of my site. If I try later, I then see FB hitting the site.

  1. Are there other conditions that would cause a server 500 error on fb.ui send?
  2. Does FB automatically fail subsequent attempts without hitting a requesting site?
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Wow. Thank you Facebook engineers for saving yourselves 5 minutes of documentation time so that I and others can waste hours. I'll report what I found. If others find additional or more accurate information, feel free to comment.

It turns out that FB tests the OG:Url it receives when going to my fb.ui send link of http://my.domain/page/?invite=xyz.

OG:url must be "http://my.domain/page". The parameter(s) are ignored. No redirects are allowed. In my case, any time as page has the invite_id parameter, it gets redirected. Checking the user agent of requests allowed detecting the FB bot. However, I found that sometimes, FB also sends the bitlybot. So you have to catch that too. Most likely things will break if a different bot is used in the future.

You get one chance to test each URL because FB will cache the result. If you try again, FB does not test the link. If fb.ui succeeded, it will continue to succeed. Otherwise it will continue to fail. This caching lasts for -- well we just have to guess.

Hope this helps. Time to get productive again...

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