1. A couple of people have interpreted the previous post as a wholesale endorsement of (or even "advertisement for") Pinterest.
I actually haven't been using Pinterest, don't have an account on it, don't see myself using it much in the future. Not really my thing. I'm also not affiliated with Pinterest in any formal way, and I don't think I know anyone who works there.
However, I have enjoyed looking at the boards friends have put together or shared links to - if you put the legal and/or ethical issues aside (which I don't think are unique to Pinterest anyway) I think it's a great new tool for curating online content. And I think there are lots of great legal and ethical ways to use it. Which is why I care whether overblown fears stop people from using it.
2. Nope, I am not saying "Pinterest is fair use."
If you don't have much experience with copyright issues, it's really difficult to understand fair use, especially with images as your main examples. Fundamentally, fair use says it is 100% legal to make copies of other people's stuff without permission or payment - SOMETIMES. Quite certainly, some of the copies people have made on Pinterest (remember, "pinning" makes copies, which immediately raises copyright issues that just linking does not) are legal under fair use. Some of them probably are not.
If you want to know if the copies you made are legal fair uses, you should learn as much as you can about fair use*, and use your best judgment. The fearmongers and spreaders of copyFUD would have you believe that you cannot possibly understand fair use well enough to make reasonable judgments about your own uses. I think better of you than that.
If the idea that people can make legal copies without paying sounds like an artistic injustice to you, you might consider thinking about all the ways fair use enables artistic production. Collage, remix, parodies, satire, appropriation art, and even photographs that reference or incidentally copy other images - all rely on fair use.
3. Yes, some copies on Pinterest are undoubtedly legal.
You can make copies of public domain stuff (note: "public domain" does NOT mean "anything that's public online". It means "a specific class of materials in which copyright has ended, or never existed in the first place." U.S. federal government documents are public domain; works on which copyright has expired are public domain.
You can make copies of Creative Commons licensed works, if the license allows it and you provide credit appropriately. You may need to learn more about what kinds of copying the various licenses allow, and how to appropriately credit, to make good calls on that.
Check out my "Joys of the Public Domain/Creative Commons" posts for examples of public domain and Creative Commons image copies: Spring Flowers; Sweaters; Rail Snowplows.
You are probably also okay making copies when the owners of the images invite you to do so - however, that can be a little murky. For example, some Etsy users were not happy when Etsy enabled a pinning function throughout the site.
4. Okay, so the Terms of Service are much like other sites; why should that make anyone feel any better?
I agree that the Terms of Service read a bit intimidatingly. But you agree to terms and conditions documents like that quite literally all the time - not just online. Most retail receipts include similar terms, as do most travel and event tickets. And yet, it is incredibly rare for companies to, for example, try to require a user to indemnify them (cover litigation costs) for a lawsuit - much less to sue their own users for breaches of the terms. (It's hard to explain why such one-sided legal agreements are so prevalent in our lives without getting pretty deep into contract theory and a discussion of how many parts of the legal system don't actually work for actual people in the real world - which is a bit beyond my ken, certainly in one blog post.)
It's also not usually attractive for people who might have the right to go after individuals in a lawsuit to do so - the "deep pockets" (i.e., entities that can actually pay damages) are elsewhere. However, copyright is actually an outlier here, and individual users do get sued fairly often - so if you believe your use of Pinterest to have been heavily infringing, some concern, and/or removal of material, may be warranted.
Finally, however powerful contracts may be, they can't functionally give Party B (say, Pinterest) rights that Party A (say, an individual user) doesn't have. So even if you uploaded stuff that you didn't have a right to upload, you have not magically given Pinterest the right to sell that image. You maybe did violate the terms of service, but the point of looking at the TOSs of other social sharing sites (or reading the backs of receipts, or even standard warranties) is to show you that you do that all the time. Not violating contracts is a good idea. But if you don't know you agreed to them, or don't understand them, there's not much you can do except try to be more aware and informed in the future.
* To learn more about fair use:
University of Minnesota Copyright Site - https://www.lib.umn.edu/copyright/useoverview
Stanford Fair Use Project - https://fairuse.stanford.edu/
Center for Social Media's Fair Use info - https://www.centerforsocialmedia.org/fair-use
EFF's Legal Guide for Bloggers - https://www.eff.org/issues/bloggers/legal/liability/IP