I know this is an old question, but I don't see a very detailed answer of the possibilities to strikethrough your text. So here's my answer:
There are several ways to do it:
~~strike~~ → ~~strike~~
~strike~ → ~strike~
Because tildes don't work on Stack Exchange (later: SE) platforms, the HTML syntax is the most correct way to strike your text. This syntax is a little "hack" to do the same thing instead of the markdown notations.
Because there is no universal notation for markdown text, the notations may be different for each platform. Here is an overview for several platforms:
- On Slack, you must use a single tilde
- On GitHub, you can use all notations
- On SE platforms, you must use the HTML syntax notations, as said above (except for comments)
The GitHub Flavored Markdown only mentions that you can use double tilde, but you can use everything.