I don't really like any of them. Try this modified version of the third. The most important event in the last year turned out to be the unveiling of a new social network.
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Mr WordyBecause thereIt came out thatThe event of (the) last year turned out to be the appearance of a new social network. -- "came out ... turned out" is awkward.
FandorinThank for the answers, guys. So "if the fact comes out" it means "it's avaliable right now and it becomes known". Is that right?
FandorinMr WordyIt
Mr WordyThis seems "unfinished" and seems to need some more qualification. In what sphere, or sense, was it "the most important event"?It was just the spur of the moment I decided upon making up a sentence. It was some kind of nomination.
Fandorin
The unveiling of a new social network (of) last year (the event) became the most important event in .ru domain and won one of the Google's nomination prizes as "the most important and expected social resource".
Is it significant to leave or not to leave of in this case? Having been provided with some information, it became better