The majority of avatars on E are photographs of models/actors or digital artwork.
These are images that are available in public locations on the internet and that is where the Fair Use thing comes in. However giving credit whenever possible is something I consider good manners and respect for another person's work. Note that this means the photographer of a model, not the model itself. (I'll freely admit to my lazy ways of not giving said credit, something I should change really.)
When the photographs are of non celebs the site actually has a rule:
No images of non-members. This does not include public-figures and semi-public figures, but rather refers to your friends and family who have not also signed up and joined the site themselves.
Which means, using random-person facebook pics is forbidden.
As for the whole idea of this outside E, there is one thing we need to consider. Posting images on the internet means making them public in nearly all cases. That's perhaps not always the legal truth, but it is the reality of it. Posting them on facebook makes them public to a large group of people depending on your profile settings. I doubt anyone uploads pics and keeps them hidden to everyone. Unless you upload images to somewhere you keep them under password protection, you have to be aware of the fact you are indeed posting them to the internet and odds are all kinds of people will see them. Using those images for various purposes may be rude and even offensive but if you want to avoid it at all cost, don't post them. Sadly that is what it comes down to.
The model etc has no legal right to a picture once it is taken unless it is for a specific reason. However, I do believe that we have a moral right to anything about ourselves, regardless of how and when and where it is produced
Its why I refuse to use photos of people and why I am strongly against the idea of media gossip and paparrazzi as I see it as an infringment of moral rights.
Celeb pics and the moral right to images of ourselves are both good points. However, they still fall under the same hat. With how society is, and we all know it, if you don't want people taking your picture and posting it all over, don't go into the entertainment industry. If you get famous, it will happen. It's not a fun fact, but it's a fact.
I will add, that should a person contact me and ask me to not use their image as an avatar or charrie pic, I would not hesitate to take it down and not use it again. Which is, actually, the answer to OP's original question ^^
Thank you for bringing this up, tozhma. I never really gave it much thought before. If Emma Watson found one of my stories and told me to stop using her image, I would. Should I also stop suggesting that any character in any of my stories look like Emma Watson? If I really want to take her feelings into consideration, I should. That would be difficult though. I am not saying I could not go without having a character look like Emma, but should I refrain from using any celebrity's appearance as a reference point? Can I say they have Emma's eyes, but someone else's smile? I am not sure how I feel about this. I am sure many actresses would be appalled by what I write about their likeness.
So, this raises another question actually.
Do you all have any idea how many characters in fiction are based on real people? Tons. Tons and tons. It's a very known method of writers to add depth to a character or even simply to use said people as inspiration. Can't we view appearances in the same way as personality and behavior? Isn't it what's on the inside that makes a person, after all?
Could this be true?:
Using pics to describe a character in a story isn't copying the real person any more than using their personality as inspiration would be.