いつの間にかすごい事になっていますね! :jawdrop:
Although I haven't checked ALL the pages with みんなさん, I found that the pages at the top are actually written by non-native speakers of Japanese (who came to Japan to study, work, etc.). In some others, "minna-san" was used intentionally just for fun. The rest appears to be typos in blogs, etc. As Nagashima-san says, it probably happens because of romaji input. As far as I know, the majority of Japanese use romaji input, unless they learn kana input first for word processing needs, etc.
I would also like to say that errors are being pointed out here because this is a Japanese language learning site (and English, too). We offer information to share with each other, which is not possible in ordinary, real-life situations. If I had met someone (a foreigner) speaking Japanese and he/she said "minna-san," I wouldn't correct the person. It is against Japanese rules of courtesy to offend someone. It is regarded disparaging rather than an offer for help. I will probably just praise who well he/she speaks Japanese and ignore such mistakes, especially since they don't affect the essential message communicated.
I just want people to know that we are all here to learn and enjoy...
Addendum: Takasugi Shinji-san is a bilingual Japanese who trained as an engineer and has a keen interest in languages (including his own). I have great respect for what he writes in his articles and the contribution he has made on the Web in creating his Japanese-language learning site.