Is there an excellent bookmark manager?

I regularly use 7 different browsing platforms. Chrome, IE, and FF on both my work and home PCs and Safari on iPad. I want to keep my bookmarks organized in one reliable place accessible from anywhere. Ideally it would be easy to export from one of my browsers to this solution to get started. Reliability is important. Backup would be a nice feature. Ease of use is important. It should seamlessly work with all 4 browsers.