Blackwell, whose company Anybots builds humanoid robots, argues that Facebook subject lines are so often just a casual "hi" that they could be removed completely. As he explains on his blog today:
Here's an idea for Facebook. Get rid of subject lines in messages. They almost never have any content in them other than "Hi". Nobody ever changes subject lines in threads, because it seems like you're trying too hard, so they're usually unrelated to recent conversation. And it makes it seem like when you're writing to someone, you have to have something to say that can be summarized.
But often what people want to say is just one sentence, "Hey, nice to see you last night at the club" or something. There's no short version of that. Getting rid of the Subject: field would lead to a lot more messages since it makes it so much easier.
While he has a point - many casual conversations on Facebook typically merit only a "hey" or "hi" subject line - removing established features is rarely a popular move, especially in this case where subject lines are totally optional. That's the key point, I think: of the Facebook mails currently in my inbox, around 1 in 10 senders just don't enter a subject line.
The subject line has value, too: if you receive your Facebook mails via your webmail account (especially on your phone), it's useful to have a subject line. For those who use Facebook professionally, meanwhile - and many, many PR pros send pitches this way - a subject line seems appropriate for these formal messages.
Counterpoint: Shorter is Faster
There are reasons to favor Blackwell's stance, however: Twitter Direct Messages (DMs) have never required a subject line since the aim is to fit the message within the length limitations of an SMS (typically 160 characters). Facebook messages, by contrast, often get split into two or more parts when sent to your phone via SMS. Facebook would likely see more casual interaction if it worked more like text messaging.
It comes down to what you think Facebook messages should be: is your Facebook inbox an alternative to email? Or is it just for more casual, SMS-like communication?