Facebook messaging - an SMS substitute?

I use Facebook quite a lot in my personal life.

I've started noticing how much it's impacted on my use of various other messaging services. Firstly, about 50-60% of incoming messages to my main Yahoo! personal email address are now Facebook notifications and alerts. Secondly, Facebook IM has now risen to about 50% of my total IM sessions, typically with a different set of people to those I contact via Yahoo! or Skype IM.

I'd also say that since the beginning of this year, I've added maybe 120 Facebook contacts, compared to maybe only 30 new friends' numbers in my phone. And there are definitely some scenarios where I'll prefer to send people Facebook messages rather than email - especially if it relates to an event, but also because the threading makes it more like an ongoingand personal conversation, perhaps because you're only a click away from someone's profile, so you always have a better idea of what's going on in their life, compared with normal email.

But yesterday saw a new example of Facebook messaging - I was due to meet a friend around lunchtime, and she posted a message on my Facebook wall saying "be there in about an hour". The thought process there is an interesting one - firstly, there's an assumption that I check (or am alerted) to Facebook regularly enough to view it as "almost realtime". Secondly, that it's not much clunkier than getting an SMS (wrong, in my view). But also, I realised that she *probably doesn't have my phone number stored in her handset*, or wanted to avoid the cost of sending an SMS. She has an iPhone with flatrate data & an easy-to-use Facebook client... so she sent a message instead.

More interestingly, it was actually a public "wall post" rather than a private message, so there's an interesting sub-communication to any other mutual friends that they should think about dropping by, if they're also in the area at the same time.

I'm struggling to think of an easy way that a mobile or fixed operator could do the same thing - a sort of casual open invitation to a large group of people, as part of what is otherwise a simple person-to-person message. It's probably the sort of thing that Twitter's good at, if I knew more than 2 people who actually use it. In this instance, nobody else turned up, but I got a message later in the day saying "oh, I wish I'd known earlier" - from someone who doesn't (yet) have always-on Facebook access.

Apart from anything else, it would be almost impossible to come up with a decent charging model for this - I reckon in this instance, 80% of the value was in the person-to-person arrangement of logistics, and 20% was in the offhanded mention for other people. Given that there were perhaps 20-40 people in my social group who might theoretically have been interested, could a telco ever price and bill this?

Despite the title of this post, I don't think Facebook will kill SMS any more than Twitter will. But I find the interplay and hidden value in different messaging platforms fascinating. It's also exactly the sort of unintended, casual use that IMS messaging almost certainly couldn't replicate.