I read an article covering that topic once, it was mainly about how apple could screw you if they wanted to- and how most sites have clauses that protect them, but if they ever wanted to they could be devious. One example is facebook. Your images can be used (all of them,, posted, in your album anywhere) for advertizing and there isn't a dam thing you can do about it. Even if you remove it from their site they can use it legally- your essentially giving up PART of your copyright where they are concerned- post and it's theirs to use and they will use it- in advertizing for the site or anything else- the tos there is pretty scary.
Why are businesses posting there? it's beyond words.
I'll try to find the article, it was pretty scary but practical, not just boasting about how sites are evil it's come to the point of lawsuits and bs that companies literally have to do it- it's just sane business practice, will they use it? Not likely, unless you become a problem. Another example is the patents, Microsoft patented both the two button mouse (likely why apple never had one) and the double-click on a mouse apparatus, they could screw a bunch of people if they wanted to (companies with two button mouses for one)... it's all come down to protection from frivolous lawsuits, think about it, if you have a law you don't use but can, there are situations that comes in handy like when someone is trying to screw your company over- but for normal people using their services, they need not fear anything... that's my take (after reading the article).
Jerrico
