For what it's worth, I have a Swedish friend who almost always posts in English, even when he's talking to his family and other people he knows in real life. He'll switch to Swedish occasionally (and Facebook's autotranslation is very understandable), but 90% of the time he uses English.
That wouldn't explain it, because we do the same in Dutch: just leave off the diacritics. Diacritics are pretty rare in Dutch, though.
Hungarian is also redundant enough that you can even replace all vowels with just one and still be understandable. Most of the meaning is carried by the consonants. E.g. "Szia én vagyok Péter, hogy vagy?" -> Szii, in vigyik Pitir, higy vigy. (sounds obviously wrong, but very understandable) Retaining the vowels but collapsing all consonants to one would be more destructive to the meaning.
Switching to English is way overblown a reaction. Two Hungarians chatting in English (unless there are non-Hungarian speakers involved) seems extremely weird to me. It may be partially that English is really foreign for us, while it's pretty close linguistically to Swedish, both being Germanic.