I have never opened the door for my coworkers to discuss weight loss. It's just not really discussed unless the person losing brings it up first.
When they start in on you, just tell them you're discussing your food choices with your doctor, trainer, nutritionist, etc. and they don't seem to be concerned so you're not either.
Also, I don't recommend friending coworkers on Facebook. Unless you're close enough to hang out on a regular basis after hours, it just gets too awkward. If you can, I'd unfriend them. If that would cause bigger issues, create a separate group for status updates that excludes them, so they will not see them.
Good luck and don't let them bother you. Weightloss is usually vastly different for men than women so if it ain't broke, don't fix it!
