If you're looking for a particular IM conversation or messaging thread you had with a friend on Facebook, you can use the social networking site's built-in message search. Because all communication on Facebook is archived, you can use the messaging search tool to search through Facebook Chat logs, private messages you've sent and received on Facebook and emails you've sent and received through Facebook's messaging system.
Sign in to Facebook with your email address and password.
Click the "Messages" link on the left side of the screen to open Facebook's messaging section.
Select the type of message you want to search by clicking the magnifying glass button next to the "Search Messages" box. You can search unread messages, archives messages, sent messages or messages received via email.
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Type your message search query into the "Search Messages" box and press the "Enter" key. Results matching your search query are displayed on the next page.