as they may upvote a meat-eater's post too

Do I understand that correctly, no conscious upvotes of posts from meat-eaters, no matter the content, even when it is for example a vegan dish?

Well, I quoted my friend's words here though I understood what he wanted to convey. If you read the post further, I've used several times the word "content", that is objectionable to vegans.

Is it according to you as a vegan unethical to have meat-eating friends?

how the society would evolve if being vegan would mean to socialize only with vegans.

I didn't mean to abstain from socializing with anyone. In fact, it's mostly non-vegans who would unfollow me because they may not see my content as friendly to them.

Where does veganism start and where does it end?

It starts from the obvious violations of the fundamental rights of survival of non-human animals that is practicable to stop by making right choices in favour of them.

Even your straight forward aproach is not so straight forward considering that space is curved according to Einstein's General Theory of Relativity

"straightforward" as meant in "easy to understand or simple, uncomplicated, honest and not likely to hide their opinions".

For example if a group eating locust in an infested area saves the habitat of a specific species, are they vegan then? Or are they vegan if they eat what the locust eat and help speed up the destruction of the habitat of that species?

Principally, vegans should not interfere in natural systems. If your farms are at a risk of being infected, it's best to use some natural repellents or some defensive net if possible.