In the general case, it seems unlikely, based on the wording (which is convoluted). In certain cases, if the president of Russia posts "My name is Vladimir Putin", that post is personal data. On the other hand, you might, based on my writing, conclude that I am from the US, and you might even conclude that I'm in Washington state, but that doesn't distinguish me from 7.5 million others, so on those grounds that is not personal data. Eventually, though, you might identify me specifically from other things that I may have said on SE.
The definition depends on two parts. First, personal data is "information relating to an identified or identifiable natural person". Any "information" provided by a natural person is "related to" that person (as is any "information" that is about such a person). The second part defines "identifiable natural person", that is, who is an "identifiable person"? Every person can, in principle, be identified by reference to some label or description of fact about them, so every person is an identifiable person, under this definition. This means that every piece of text that refers to an individual (not even text which can identify the person) is "personal data". Obviously, any individual can be uniquely identified by some collection of identifiers; the problem is that the wording of the law does not explicitly say "using that supposed personal data". If I mention that I have a relative named Knudt, that would technically be personal data: I've given information that relates to a person, though you have no idea (and could not possibly figure out) who that person is.
Another term that the regulation defines and uses in a few places is "pseudonymization", which is defined as
the processing of personal data in such a manner that the personal
data can no longer be attributed to a specific data subject without
the use of additional information, provided that such additional
information is kept separately and is subject to technical and
organisational measures to ensure that the personal data are not
attributed to an identified or identifiable natural person
The point of interest here is that this says that "personal data" which cannot be attributed to an individual is, nevertheless, still personal data. I think the most important part of the regulation is art. 6, which defines lawfulness of processing, especially para 4., which allows consideration to be given to safeguards such as pseudonymization.