Indeed, I have seen this behaviour for at least a week. It seems to somehow be related to geo-located DNS (akamai?) since the timeouts and RSTs I see in the traffic are from IPv6 akamai servers, not facebook itself.
I tried de-peering from a number of networks to try and isolate it and it was not transit-specific. I stopped questioning my network when I saw a thread on Sixxs and tunnelbroker about this very issue.
Hopefully it won't persist as right now as having Facebook intermittently connect is likely hurting deployment slightly.
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Your IPv4 address on the public Internet appears to be 213.65.35.162
(TELIANET-SWEDEN TeliaSonera AB)
Your IPv6 address on the public Internet appears to be 2001:2002:d541:23a2:11fe:f37f:1493:bb73
(TELIANET TeliaNet Global Network)
It appears that you use a non managed tunnel mechanism for either IPv4 or IPv6. [more info]
Good news! Your current configuration will continue to work as web sites enable IPv6. [more info]
Your browser is blocking the test urls. We will try alternate methods, but they may fail to show your IP address; and may affect the quality of the advice given. [more info]
You appear to be able to browse the IPv4 Internet only. You will not be able to reach IPv6-only sites.
IPv6 connections work, but connections using DNS names do not use IPv6. For some reason, your browser or your OS is not doing IPv6 DNS 'AAAA' lookups. [more info]
Your DNS server (possibly run by your ISP) appears to have IPv6 Internet access.
Your readiness score
0/10 for your IPv6 stability and readiness, when publishers are forced to go IPv6 only
Click to see test data
(Updated server side IPv6 readiness stats)
Copyright (C) 2010, 2012 Jason Fesler. All rights reserved. -- r1190
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We're actively investigating the v6 issues. We need more data
though. If you're experiencing problems, please email me a
tcpdump/pcap or any other debug data you think will help.
I was experiencing problems, and disabling ipv6 seemed to fix it,
and have also been experiencing sometimes slow speeds to google so
didn't know if it was concurrent facebook/google issues (which I
suspected) or a general ipv6 issue.
So I reenabled ipv6, and did tcpdump and it seems to be behaving fine now,
although slightly slower than ipv4 even though ipv6 seems to pick a closer
data centre than ipv4. Both of which are further away than they
used to be. (entry point lax, destination seems to be frc1 for ipv4
and www.v6.facebook.com, and prn1 for ipv6 www.facebook.com.
Is there some url that could run a regular curl to to see if it times out
off/on?
Another concurrent issue, is it seems images are getting slower and slower
over time. It seems for some reason that https is forced, even if disabled in
profile settings and that cdn usage keeps varying over time, it used to have
urls like photos-a.ak.fbcdn.net but now it seeems to have fbcdn-sphotos-a-a.akamaihd.net. Curiously photos-a.ak.fbcdn.net still looks
up with < 1 msec ping rather then ~190 msec ping.