I can’t vouch for Facebook since I’ve never had an account there.
I used livejournal for years and it was nice, though these days it feels deserted, because so many people have left it over the years and prefer using other blogging platforms nowadays. So if you want to reach a lot of people it might not be ideal.
I'm a big fan of tumblr and have a blog there and post my fics (drabbles/oneshots) there too.
The thing about tumblr is that it's very versatile, you can make it anything you want to be. And not just your own blog but your dashboard too. Your dashboard will only show posts from blogs you follow, so really it can be anything you want.
(For me it's art and books and pictures about nature and architecture. It's inspiring quotes and writing advice and fandom stuff. So like I said, anything.)
There are many themes from which you can choose a suitable one for your blog and you can even customise them further with CSS it that's up your alley.
On tumblr you have one main blog. The url of your main blog acts much like your default username. It will show to people when you follow them, when you like a post, when you submit to another blog, or when you send someone a message (called “ask”).
But you can also have sideblogs. They’re not necessary as you can certainly get by with a main blog alone, but sideblogs can be convenient if you want a blog that is private (as they can be password protected), or if you want to have blogs that have different themes (e.g. separate blogs for different fandoms).
Also worth mentioning is that there is a browser extension for tumblr called xkit, where you can choose to install different apps to make using tumblr more convenient and give you options you wouldn’t normally get like mass-editing posts on your blog, blacklisting certain types of posts or selecting to see the tags people have added to reblogged posts (which is nice for example if someone has made comments about your post in the tags).
It can be a hassle and a timesink.
Whether it is worth the time and effort, I don't know, I think that's something only you can answer, since it also depends on what you want from your blog.
I just know I've certainly enjoyed my time there and it's allowed me to take part in things I might have otherwise missed out on, like the SessKag week they hosted in August. Also one of the fandoms I write for is much more active on tumblr than it is on either ff.net or AO3.
If you're interested or have any questions or confusion about either tumblr or livejournal, feel free to ask.