It frustrates me when I see businesses or interest groups that focus their entire online presence on one platform that they don't control.
"Join our Facebook group to find out more!" or "Follow us on Twitter for updates!"
Then, once their group has grown to a certain size, they find out that they are only able to reach 10% of their audience unless they upgrade to the premium package, or pay a fee per 500 users and they have no option but to capitulate or slowly try to move their users to a new platform (which might not even be possible if you can't reach the audience in the first place).
Not to mention the censorship that some groups may face, whether that's objections from inside their own group, or clashes with the site-wide policies of whatever company they are using to host their communications.
The advice in 2019, is the same as the advice from 2005. Your website should be your main point of contact no matter if you are business, a community or if it's a personal page.
Once someone has been directed to your site, there you can point them to your Facebook, Instagram, Twitter or whatever social media you are using this year.
I'd recommend the addition of one extra line though. Something like:
Reach us on IRC at #yourgroup on irc.yourdomain.com
Then, in this channel, you can summarise the activity on the rest of your platforms in one place:
This information leads to discussion and the instant nature of IRC allows the community to develop in ways that are much more difficult to achieve on forums etc.
Due to the nature of IRC you can immediately see who's active in your community and it bypasses the restrictions on contacting your customers/community introduced by the GDPR because you aren't emailing this information, and instead, the users are coming to your channel precisely because they want to be informed of what's going on.