Everyone is a pronoun which means everybody, all the people who make up a group. Everyone refers to a group of people as a whole. The words everyone and everybody are interchangeable.
Every one is a phrase which means each person who makes up a group of people, each individual. Every one refers to each individual in a group. The phrases every one and each one are interchangeable. Interestingly, the word one as used to refer to a person, or one of those people, was originally pronounced with a long “o”, demonstrated in the word “only”. It wasn’t until the fourteenth century that one began to be pronounced with a beginning “w” sound, as it is today.
Examples
Facebook blocked the phrase “everyone will know” for more than a day, but users are now once again able to post the seemingly innocuous three words. (The Huffington Post)
There have been many instances where someone crossed a line, and we were tempted to respond with a new rule that applied to everyone. (Forbes)
Back then the national capital had only two channels and just about everyone watched Star Trek repeats on Saturday afternoon. (The Australian)
“I encourage everyone to read each of this year’s shortlisted books and to enjoy the mystery, honesty, intrigue and entertainment of each of the stories, and then to cast their vote in the People’s Choice, which was initiated last year and will be presented again in 2015.” (The Sydney Morning Herald)
“We are going to kill every one of these bastards we can find, because if not, they are coming here,” he told viewers during the second GOP presidential candidate debate Wednesday night. (The International Business Times)
These maps also reveal a larger national pattern: Among the top 25 highest-earning counties in the U.S., every one of them lies outside a major city. (The Washington Post)
Every one of us is surrounded by a “microbial cloud” containing millions of bugs which is almost as personal as a fingerprint, research has shown. (The WEstern Daily Press)