1) Everyone is doing it.
I'm sorry, but as soon as I hear that as a reason for doing ANYTHING, it is an automatic turn-off. Only occasionally is something that "everyone" does a good thing. E-Mail, for example. Of course, I don't recall that ever being used as a reason to get e-mail.
2) It's redundant.
If you want to contact me, I have E-Mail (two work and three personal addresses that I check on a regular basis), cell phone, landline phones (work and home), a wide assortment of IM options (Skype, AIM, Yahoo IM, Google Chat, ICQ, etc), and, for some of you at least, I even see you on occasion. Why on earth do I need yet another means of communication?
3) It's impersonal.
Unless you send a direct message (you can do that on Facebook, right?), messages are posted for any and all comers - you are not talking to me, you are posting a flyer on a street corner. And if you are going to send a direct message, why not just use E-Mail? It's easier, and I already have it. Or IM? Makes it much easier and faster to hold a two-way conversation. Or even call? Then I get not only your words, but your tone of voice as well - helps avoid mis-understandings that way.
4) It's rude
With every other form of communication, it comes to me. When I get an e-mail, IM, or phone call, I get some sort of alert notifying me that someone wants to communicate with me, and the communication is instantly available, with no more effort on my part than picking up the phone or clicking the window. Not so with Facebook. As I already mentioned, posting on Facebook is akin to tacking up a flyer on a street corner. So in essence with Facebook you are expecting me to take time out of my day, perhaps even several times a day, to "walk" over to the street corner to see if maybe, just perhaps, someone posted something there worth reading. You wouldn't expect everyone to come over to your house whenever you wanted to talk to them, would you? Why do you think it's ok to expect them to go to "your" website?
5) It's public
Ok, admittedly there are some things that you want the public to know about- if you are selling something, for example. Most of the time though, that is not the case. When I "talk" to someone (through whatever means) I expect my conversation to be with them, and for them to be giving me their undivided attention. I don't want to talk to everyone in a crowded room, and if someone is trying to talk to me I don't want THEM to be trying to talk to everyone in the room- I want them to talk to me. We all hate that person in a crowd that talks louder than everyone else so you can't help overhearing their conversation. When you post on Facebook, you are that person. Sure, you may be able to limit the size of the crowd by making your profile private, but's it's still a crowd.
6) It's unreliable
When you post on Facebook, you are one of potentially hundreds of other people also posting on Facebook. There is no guarantee that I will see your message amongst all the others vying for my attention, and unlike E-mail there is no persistent indicator that I have not read yours. I may see it, or I may not. With every other method there is an indicator for your message specifically that I have not read it that remains until I have. You can be much more assured that I actually get your message.
7) It's permanent.
When you post something on Facebook (especially if your profile is public), it is there forever. You may be able to delete your profile eventually if you jump through enough hoops, but you will never get rid of all the traces. When I delete an e-mail, it's gone. There MAY be a backup copy on the mail server for a while, but that's it, and they have strict privacy practices preventing that backup copy from being used for anything other than restoring in the event of a crash. Even that will go away sooner rather than later.
8) It has no value for me
This is, on the face of it, very similar to #2, but from a slightly different angle. Ok, let's ignore for the sake of the argument all the other communication methods I already have. What unique value proposition does Facebook offer me that I can't get better somewhere else? Games? Please. Like I have the time or desire to play stupid little flash games on some website. Communication? Nope- just about every other form of communication out there is superior in one or more ways (yes, I can list them). Something else? Not that I've heard of. When you come right down to it, Facebook gives me nothing, while requiring a commitment of my time and energy. No thanks.
9) The news.
I mean, really, have you seen all the controversy in the news lately about Facebook privacy practices? Yes, they claim to be addressing the issues, but what was the last time you heard ANYTHING about E-Mail privacy issues? Or phone?