This seems to come up a lot on writeblr discords! As a user of both tumblr and twitter (more writeblr on the former and more casual/personal on the latter) here are some things I know or think about Twitter, writing Twitter, and the comparison between Twitter and Tumblr for writers.
Tweets have a much shorter time in the spotlight, unless they go viral and/or spark discussion.
That said, Twitter isn't built for in-platform long-form, and your best bet is to use it to redirect traffic and attention to your links elsewhere. In writer terms, Twitter is for pitching and marketing your story, not directly sharing the text.
Which brings me to: writing Twitter is best for (but not exclusive to) writers with finished or almost-ready-for-viewing manuscripts. It isn't best practice to pitch a story you aren't finished writing. In fact, it's polite to have a polished draft ready, not a rough one.
I mentioned on a discord discussion that "Writeblr is where you go to get writing, writing Twitter is where you go to get read." and someone agreed with me. I'm as comfortable a Twitter user as a Tumblr user, but most of my writing activity is here because I'm too early in the process to be talking about my book with agents and editors. I need to have something substantial to show them if they ask.
Writing Twitter has a combination of novelists, poets, short fictionists, and journalists. If you want to be picky about your networking niche (which is valid!), look through a person's bio and past tweets to see what category they tweet most about to see if this is someone you're interested in following.
Lastly, Twitter requires active interaction. You definitely have to put yourself out there way more than you do on writeblr. Seek out public tweets and reply to them.
Oh and it's probably a given, but just in case it needs to be said - if you intend to use writing Twitter as a career-building / networking venue, then using a human profile picture and a proper name (use a pseudonym if you must) will make people more comfortable interacting with you and taking you seriously. (Again not a hard and set rule.)
That said, here are some places to start reading, participating, or studying to get a feel for writing Twitter:
General hashtags where discussions happen: #writingcommunity, #amwriting, #amediting. I know there's a more niche #amwritingfantasy, #amwritingfiction (because those are my genres) but have not checked if it exists for other genres. I don't particularly follow #authornews but I see it get used here and there.
Diversity hashtags - not much discussion, mostly a mixture of discovery features and opportunities. Again, these are limited to hashtags on my radar, but if you ask around you'll definitely find a hashtag for your community. It won't always be active, but it'll be there. #writersofcolor, #writersofcolour, #SouthAsianWriters, #writeLGBTQ, #readwomen. I've encountered #blackwriters and #blackauthors but have not checked for which is preferred, if any. #ownvoices is a pretty universal hashtag, and is particularly active on Twitter.
If you're from a major city in the States, I'm willing to bet there's a hashtag for your city or state if you dig around for it.
Hashtags and communities not specific to writing, if you want to get a feel for how else Twitter is used: #ActuallyAutistic, #NeurodiverseSquad, #VisibleWomen, #PortfolioDay
Now if you're ready for the full force of writing Twitter, below is the real good shit:
#amquerying and #MSWL (stands for Manuscript Wish List) is where agents and publishers announce that they're looking and share what they're looking for in submissions.
@PitchWars is one of only three accounts I'll recommend (for now). They have two important projects you should be watching: #pitchwars and #pitmad, a mentoring program and a manuscript pitch party respectively.
The second account you need to be following is @AuthorMentorM and their hashtag is #AuthorMentorMatch.
The third is @DVpit_ and their hashtag is #DVpit.
Some last bits of advice:
Diversify who you follow. Follow agents, major publishers, indie publishers, literary critics, small presses, even writing advice accounts. Your fellow writers are probably following those accounts too.
If you really feel like going on a follow spree, look for one account you like (like an admired author, a dream publisher) and scroll through their Follower/Following lists.
When in doubt, tweet a question asking help and use the #writingcommunity hashtag. You will get assistance.
Twitter as a space is not as generous with trigger warnings. The more popular and generalist hashtags (stuff like #ownvoices) can and do get used for political arguments instead of intra-community discussion. Browse at your own risk.
That's as much as I know - and I'm not even an active participant on writing Twitter yet. I just find Twitter search much easier for discovery than Tumblr. If anyone has any questions about Twitter, writing or otherwise, I'm happy to answer them. You're free to ask me for my Twitter too, and I can add you to my "writeblrs on Twitter" list if you like.