Adding a hashtag button for Twitter is pretty straightforward with the Hashtag button but for Facebook and Google Plus you need to get creative and use a prefill text to toss in the hashtag (In my case #FedoraAppreciationWeek2k18). Plus you need to register your application to get a client ID for both the FB and Google share buttons.
Don’t prefill any content in captions, comments, messages or the user message parameter of posts unless (a) it is a single hashtag in a post shared through our Share Dialog (but not via our APIs), (b) it was created by the person using your app, or (c) it was created by a business whose employees use your app to administer the business’s presence on Facebook.
To make it easier for the FB crawler to fetch the relevant content from the site (instead of trying to guess what the header and body for the post should be) we use Open graph tags.
You can even check what it might look like using the Sharing debugger.
I decided to go with a simple link to the twitter handle. But there is an official follow button available. You just can’t customize it much.
We do have a follow button for Google +. And this one does not require a client ID.