Hashtags are everywhere. You could say that they are #ubiquitous
Not just on Twitter
They started on Twitter, but now they are just as common on Facebook or Instagram. The problem is that on a Mac, it's not as straightforward as you’d think to type the hashtag (#) symbol.
You can always copy & paste the # sign from another tweet or Facebook post, but that is not fun.
Where’s the Hashtag symbol?
The hashtag symbol on a UK Mac keyboard can be made by pressing the Alt Key (more commonly called the Option Key on a Mac), and the 3 key at the same time
If your keyboard is a US layout, then the hash symbol is most likely to be found by pressing the Shift Key and the 3 key at the same time