Okay so I've been able to replicate this on my mac now so it's not client specific, just by viewing the facebook activity feed.
Nov 17 23:34:07 ubnt kernel: [wan_in-6-default-D]IN=eth0 OUT=eth1 MAC=80:2a:a8:8d:3b:44:0c:a4:02:5c:10:01:86:dd src=2a03:2880:f01a:0005:face:b00c:0000:0001 DST=2a02:0c7f:7013:d101:bc7e:76c1:e09e:b2f7 LEN=136 TC=0 HOPLIMIT=57 FLOWLBL=0 PROTO=TCP SPT=443 DPT=56761 WINDOW=114 RES=0x00 ACK URGP=0
I could change the tcp timers on the router but these aren't exactly long connections. Is anyone else experiencing this?