No they aren't. Nobody sat down and said "how do we design Instagram to be maximally addictive". They just said, hey, look, I made some neat photo filters and someone else said, hey, love the filters, it'd be neat if I could share these photos with my friends. Badaboom, a social network.
The fact that so many people spend so many hours on Instagram is nothing to do with the software itself and everything to do with how people use it. To be brutally honest here, in my experience it's 90% girls who obsess over Instagram. I know very few men who use Instagram at all, let alone post regularly. Women use it so much because the posts they configure the app to show them consist mostly of girls boosting each others self confidence by telling each other how beautiful or successful they look in each photo. Equally important - what's not there: no politics, re-sharing of links that someone might expect you to read or care about, and in general nothing that would require any mental effort at all.
Nobody designed Instagram to be full of trivial ego pumping, it's a neutral photo sharing tool. There are no dark patterns, whatever they're meant to be, unless you believe that the absence of a "share this link" feature is a dark pattern. It just so happens that this is how many people choose to use the tool.
In the same vein, nobody designed Twitter to be mentally less taxing than other social media sites, it was just that due to technical limits SMS restricted your character count, and Twitter inherited that limit, and limiting the size of a post sets a very low but very consistent quality bar on tweets, such that people who would have been intimidated by blogging full sized articles can now engage in online discussion without feeling out-competed.
And why not? The attitude you display here shows a great feeling of superiority: people don't like Twitter or Instagram because they like the culture on these networks, instead it's because they've been "addicted" by "dark patterns" beyond their comprehension of control, in some sort of vast Silicon Valley conspiracy that is mysteriously unsupported by any sort of paper trail or emails showing discussion of these malign intentions.
Please, grow up and just let people enjoy communicating in whatever way makes them feel good.