They aren't fair use, but I doubt you have anything to worry about.
First, in regards to fair use of copyrighted material, simply altering a copyrighted image is not fair use. Doing something like that is actually called creating a derivative work, and derivative works actually fall under the copyright of the owner of the original. There's a lot of talk online about fair use because the derivative work was "transformative", which I suppose means that the person who made the derivative work did so in such a way that they feel it is completely new. However, at least legally, that's a shaky claim that can only really be assessed on a case-by-case basis. Fair use, as legal term, as written into copyright statute, is very specific, and has narrow scope in what's really allowed. Derivatives are considered fair use only for purposes of education, criticism, or parody, and even in those contexts, it's not a carte blanche.
In reality, "transformative" fair use isn't really a thing. When people talk about fair use that way, what they're actually talking about is making enough changes to source material that the derivative work is now both recognizably its own thing, and that its existence won't have a negative financial impact on the copyright of the original. It's all very complicated, and can only really be figured out on a case-by-case basis. The bottom line is this: most stuff that people talk about being "transformative" can't be defined as fair use, which is its own term in copyright law, with its own set of criteria to meet; also, most stuff that people talk about being "transformative" is probably truly copyright infringement, but it isn't in the best interests of the original copyright holders to press their claim.
Second, what you're dealing with is trademark, not copyright. A trademark is quite often much more closely protected, and more strongly defended by its holder. A trademark is the thing by which an organization is quickly identified. All of your icons contain company trademarks (registered trademarks, by the way). If you were to change the actual marks (the twitter bird, the instagram camera, the chaturbate C) then it might be problematic. But you didn't change the marks. All you did was (I assume) put them in shiny boxes. Considering these linkback icons are explicitly intended to advertise the sites, using those marks in icon boxes that you've made is fine, as long as you haven't changed the marks themselves (which you haven't) and as long as you don't intend to use them in a way that misrepresents the owning companies (which, if you're using them for their intended purpose, you won't be.)
TL;DR: You're fine. It's not fair use, but it doesn't have to be. This has to do with trademark, not copyright, and as long as you don't change the marks and don't misrepresent the companies through the use of their marks, it's kosher.
