Honestly the simplest option is sometimes the answer. Google docs...
It meets all your requirements. Markdown interop is very good. Always backed up and in sync. Supports offline access. Really easy to export everything if you really need to.
Google Docs doesn't do classic "wiki" things like bi-directional linking. Search, ironically, is also a bit of a mess.
Google Docs was built as a MS Word competitor and that's what it does best. I love Google Docs and I use it every day, but this is one thing I wouldn't use it for.
