Some parents are very negative and seem to take glee putting that out there. It's always been like this though, it's not a social media thing - back when I grew up, it was a way a lot of mothers socialised together, going on about how horrible parenting was and warning the kids to never do it (while also cursing them "I hope you have one just like you").
Part of me is glad people can be open about what they're struggling with and get support and compassion. Part of me finds it uncomfortable at times. I don't like it being done around the kids - it led to a lot of horrible feelings growing up, I can only imagine how kids who are being filmed while their parents discuss how horrible parenting them, knowing it's going to be shared with the world are feeling.
Sometimes it does feel both online and in-person that people are trying one up who is having the worst time parenting. I saw it a lot growing up, still see it now, it seems popular when someone is discussing considering having kids for them to get piled by parents on all the horrible things, and how they need to 'enjoy X while they can.', I find it very draining. I imagine the opposite is equally so, but I haven't seen it so don't have that experience.
I don't think I ever had the biological urge, having kids was a decision my husband and I came to after a lot of conversations we had about it before we got engaged and decided to live together permanently. Other people's chat about it wasn't really part of our decision making, it was more do we agree to have kids and what are our values and priorities around them and other parts of life. I fully think I would be equally if very differently happy without kids, and I'm open that it was something I chose because of those conversations and our agreements on it that ended up working (I told him at the time that if he didn't keep up with that, I'd stop at one. We have four).
Of those concerns, first varies a lot, but there are plenty of women who get into the best shape of their lives later in life after kids; being tired all the time from kids I'd say was only really when they were tiny and I was breastfeeding at night, if we didn't have time to do anything - websites like this wouldn't work; this is down to the couple - it can happen with or without kids, though I agree with others it's hard to tell before having kids how much will actually end up on your plate.
As others said, what you interact with is what you get on many social media platforms. If you try looking up more positive content or block that content, you'll get better.