Adapt this:
http://forum.mikrotik.com/t/print-built-in-routeros-variables/139713/6
And have a look at the wiki linked to.
update, adapted it for you:
:put ([:pick [/ip firewall address-list print as-value where list=Facebook address=31.13.67.20] 0]->"timeout")
:pick start his search for the word timeout at position 0 (zero) and as-value returns the value of the found field.
Update 2, because this timeout is also show without using print there is a simpler way:
:put [/ip firewall address-list get [find list=Facebook address=31.13.67.20] timeout]
Find selects the searched line and get with timeout (name of the field) returns then the value.