What happened to subject lines in messages?
The new Facebook Messages is more informal than other messaging systems. Now your messages are included as part of ongoing conversations, which means that some aspects of traditional messaging systems (such as subject lines and the Sent messages folder) are no longer part of your experience. This makes your conversations easier to read, especially when your messages are mixed with your chats and texts.
If someone sends you a message from a system that still includes subject lines, the subject line appears in bold as the first line of the message within your conversation.
If you’re looking for a message and you can’t immediately see it in your main messages view, try finding it using search. You can also easily view all your sent messages.