Should Facebook remove subject lines from messaging system?

No.

Facebook messaging is like email... and if you get a lot of messages, subjects can be helpful.

The comparison to Twitter doesn't really make sense. It is Facebook status that is like Twitter (and has no subject requirement). Facebook's inbox is not like Twitter.

You're right. Facebook is not Twitter. Twitter is what happens when you eliminate everything but subject lines. It's a platform for transmitting social MIME headers.

I'm undecided because, while Facebook's message is like email, it doesn't need to behave identically.

For example, the mailbox view could simply display the first line or so of the message's content to give the reader context.

So whip together a Greasemonkey script and put the choice in the hands of the savvy user. Not the most elegant solution, I know, but it's clear there's a difference of opinion so taking either road would leave someone out in the cold.

Yes.