Yes I think I do a great deal of interneting in English. If I knew only my native language, I´d have a much more limited acces to informations. However, I don´t think it is detrimental to my own language skills, that´s a bs. I have seen ppl though who have lived in English speaking countries for most of their lives and when they were supposed to speak czech again, they were forgetting words and instead inserted english words. It seems to me a bit retarded, or I have the impression they do it on purpose because they think it makes them appear more classy or something. Some ppl think it is cool using many english or foreign words, but to me it looks terrible, as if they didn´t know how to translate it properly. So when I speak my language, I rather avoid using foreign words in it if I can use czech words instead. Not that there was something wrong with English, not at all, only it looks terrible when it´s used in improper place.
So I don´t think it is detrimental to my own language skills, but is true that when I don´t use some FOREIGN language for a long time and then come to the country, I have difficulties getting "into it" again. But it settles within several weeks because I have to use it. But no matter how long I am away from home, I never forget my native language.