I’ve seen that alot of people are confusing the bounce rate with returning visitors. But those are two diferent things.
Returning visitors is the proportion of visitors (in total a defined period – usually a week) who visited and came back.
The number of returning visitors is very hard to reach 70%, only for small sites, visited friends in particular, without promotion, or with a very select audience.
I’m not documented, but i guess that any site that appears in Google it can hardly get a high number of returning visitors, no matter how faithfully would be the public.
Bounce rate is calculated based on one’s time: day, week, month, year etc. in Google Analytics.
Bounce rate is the proportion of your site visitors who are leaving the site without clicking anything.
The average of bounce rate is 60% but it is much better to keep it below 50%