User requirements can be very complex. Software that attempts to solve these tends to be complex as well. It also tends to be sufficiently useful that it can be monetized, which leads to commercial implementations where added complexity is generally a positive that results in more users having more of their needs met.
Not always of course, and yes software gets bloated and gross as well. But in general, the added complexity of software over the last two decades has resulted in software that is easier to use, more forgiving, and therefore more useful in terms of solving more problems for more people.
Simplicity is indeed a positive factor for an ecosystem, but not a requirement. Look at Salesforce.
As for FOSS specifically, simplicity is surely much more important, but that almost seems tautological. FOSS is typically written by people who do it for the joy of programming and to solve their own needs, which is wonderful, but not lucrative. This the chances of a FOSS project being successful are highly correlated with it being something (a) useful enough that many people want it but also (b) relatively simple enough that most engineers could roll their own if they wanted to - and thus they have the time and skills to maintain it for free.
FB is adding features to try to keep users on the site, and as a competitive moat, but in the process, they are making the user interface increasingly hard to navigate... There's just so much going on. I know that multiple times, I was looking for the place on my profile, trying to find where I could see all the photos I'd uploaded, and not finding it.
I'm not sure FB videos is really good for users. YouTube offers a much better service, that's searchable from the outside. FB dating is currently terrible compared to OKCupid and even Tinder. It definitely looks like bloat.
That being said, I agree with you about OSS projects being simple by necessity. I've had to give up a successful OSS project I started because reviewing pull requests was becoming a second job on top of my full-time day job.