Social networks largely unknown outside of Russia? We have'em (vk.com, ok.ru). Reddit equivalent? See pikabu.ru. IMDB? See kinopoisk.ru.
There's a Russian browser (Yandex.Browser), Russian map service (Yandex.maps), tons of Russian e-mail, hosting and cloud services, Russian Spotify (Yandex.Music), Russian Netflix (several of them, actually), Russian Uber (Yandex.Taxi, which actually owns Russian Uber).
You'll see lots of Yandex services here, it's sort of Russian Google (except it predates Google by a year or so). Yandex's primary business is search and advertising, but just like Google, they diversify a lot. And even in primary area, they sometimes manage to beat Google. Yandex's reverse image search (when you upload the image to search for similar ones) is FAR superior to Google's.
And there's a lot of unique Russian content on global sites like Facebook, Livejournal (owned by a Russian company nowadays) or Wikipedia.
The Russian Internet seems to be an overall great place to find information on old devices, old software, and the like. http://sht-rajvo.narod.ru/index.htm is a retro-looking site about retrocomputing, it has many articles from computer magazines circa 1990. Also worth noting that due to Russia's traditionally "relaxed view on copyright" it's not hard to stumble upon a site that has direct download links to e.g. versions of MS-DOS or Windows 3.1.
dic.academic.ru allows you to search through several dozens encyclopedias. And bigenc.ru adds onother one (the largest and the most recent).
fantlab.ru is the best site dedicated to sci fi/fantasy literature (it is IMO 10 times better than goodreads or librarything). There are also a lot of site dedicated to literature like proza.ru lib.ru litres.ru feb-web.ru www.obshelit.su etc.
Besides habr, forum.ru-board.com ixbt.com cyberforum.ru overclockers.ru 3dnews.ru are very popular sites dedicated to hardware/software/coding.
There are a lot of sited about video games like old-games.ru goha.ru stopgame.ru riotpixels.com as well as a streaming platforms like goodgame.ru
rutube.ru exists for many years now but it's crap.
There are several sites dedicated to popular science like elementy.ru arhe.msk.ru gramota.ru histrf.ru
www.intoclassics.net and www.classicalmusicnews.ru are popular for those interested in classical music. www.darkside.ru and rock.ru for rock music.
forum.awd.ru and otzyv.ru are popular travel sites.
There several general purpose forums like forum.rcmir.com www.e1.ru/talk/forum/ In general, classic forums are still very much alive in runet (hell, even LJ is still alive) and there are a lot of niche forums you could visit.
There are more than 100 news sites, but the quality is quite average (like everywhere else). meduza.io ria.ru rbc.ru tass.ru inosmi.ru for example. sports.ru and championat.ru for sport-related news.
ozon.ru is now a russian version of amazon.
And obviously there are a lot of pirate sites from rutracker to flibusta to libgen.
But I do love the old usenet-like interface with thread trees. I wish more message boards still used a similar interface (although it is a pain to use on mobile).