Fortnight is certainly in common everyday use in the UK. So much so that the use of "two weeks" instead of "fortnight" would be a possible pointer to a speaker for whom English is not his first language. It is a little like saying "שתי שבועות" instead of "שבועיים" Another common English language pitfall is to say "two times" instead of "twice". We all understand exactly what is meant but a British native speaker simply wouldn't do it. However, the use of "thrice" to indicate three times is now considered old fashioned and is rarely used these days, except in literature and poetry. What the Americans do, though, I do not know. There are numerous small, but significant, differences between the two branches of the language.