You'll need to define Social Media. For some WhatsApp (and the like) are SM, for others not.
That's what you
want - you might want to make sure that the recipients understand that too!
There are lots of good suggestions already but most are going to require the recipients to download/log in or at least navigate to certain websites.
The challenge for you will be sending the appropriate link(s) if they already do not use any of the main electronic communicators.
For any folk especially reluctant to go online you could start in advance of your departure and help them through the process.
It may be worth considering if one link will allow someone to follow the whole journey or if they will need to receive new links to see the new "content",
It may also be a good idea to clearly set out your purpose with this, otherwise the lack of an update or two or "deadlines" missed might lead to a stream of calls/texts and unnecessary anxiety. You
might have done your research but they might have some strange ideas about where you're going to!
In fact, there's a good argument to be made for an almost complete "switch off" and a "no news is good news" policy. Perhaps one family member is nominated to be the contact and they get a regular message to say that all is good. The news filters out from them.
Is there anything to be said for a good ol' postcard?
Just on this, whatever about a Blog, my travel experience has been heightened significantly by developing the habit of taking notes as I wander along. On a modern phone, most photos are geotagged so I can see exactly where I was when I took them, notes and voice memos then can flesh out the detail of that one photo (normally conveniently time stamped).
Maybe it's me, but it's amazing how a daily routine like on a Camino can become blurry and confused over time. (In fact, there's a lot to be said for not being particularly bothered by what day of the week it is!) A daily flick through some photos and a quick read (or listen) of notes at the end of the day can really "bed in" the memories.
Not for anyone else. For you.