It's not possible that Pidgin causes it directly: this private address range simply will not be routed. Moreover, my private networks use the 192.168.0.0/16 range. This 10.0.0.0/8 range could never have been from me. It also makes sense for Facebook, given the size of their internal network, to use the larger 10.0.0.0/8 range for internal addressing (though it'd make even more sense to use IPv6).

Perhaps Pidgin (or any xmpp chat client) triggers some event that does this, but I don't know what or why.