how many messages can fb hold in inbox?

hi guys,
i wondering how many messages can hold in fb inbox, from the same person pm/chat history.
and some of my oldest pm history is deleted somehow (automatically).. can some one tell me how that happened please?

[my pm history is covered more than 7200 now with my frnd/gf ]

lately i checked my previous messages with my gf and i found that most of the old messages deleted without my knowing/permission..

is that possible to happen automatically?
and is there any way i can stop that auto deleting?

thanks u friends..
peace.

When Facebook upgraded, it deleted all old posts. Facebook deletes them to make space on their servers. If you want to keep all your conversations, go to Account > Account Settings > General > Download a copy of your Facebook Profile.

thanks for reply.
so if i download my profile copy, do i gonna recover/see all the deleted messages?
thanks..

No, the deleted messages are gone and you cannot get them back.

oh..
anyway thanks for ur time and sharing..
appreciate it..