However, just because FSF thinks linking is enough to make derivative work doesn't mean it necessarily is. I have come to think that GPL isn't good license because of how horribly underspecified it is in that regard.
Exactly. In the EU "linking two programs does not produce a single derivative of both (each program stay covered by its primary licence)" because Directive 2009/24/EC allows this: "any portion of code that is strictly necessary for implementing interoperability can be reproduced without copyright infringement." https://joinup.ec.europa.eu/collection/eupl/matrix-eupl-comp...
On the other hand if you view the license as a contract, I believe it can make arbitrary demands about the license of other (non derivative) software.
edit: actually even if you consider it a contract it probably can't prevent you from doing something that is allowed by the law, so it can't affect the license of other software. I'm not a lawyer.
For example, in principle a screenshot would be a perfect example of a "fair use" for citation/reference. In practice, lwayers tend to disagtee - as "a small part of a work" is something horribly unspecified.
Of course, if you are Google/Facebook/Apple or so, you can do as you wish. (Unless you are at risk of a legal war with another corporation.) So because they use it in a given way does not mean that you will be able to defend yourself the same way.