> Do you believe that young girls are affected differently than young boys by social media?
Yes.
Women are taught from a young age how they must act, dress, etc. What they should look like, that they should fit into a size 0, be <100 lbs, start counting calories when they hit puberty, etc.
Keep in mind that every attempt at a teenage boy's magazine has flopped, while teenage girl's magazines (at least until the decline of magazines and print media more generally) were quite popular. And this is in large part because there are a lot of expectations placed on girls that were never placed on guys, so these magazines helped guide a lot of young women through puberty (whether they guided them well is a matter for another discussion entirely, but they certainly did hold a lot of influence).
While there certainly are images of the ideal man that societies have held up over time, they don't have the same weight as expectations for women. If you have a dad bod, nobody is going to be glaring at you at the beach for showing it off, whispering about how "un-masculine" you look, that you need to show a little modesty, but a woman who goes to the beach with hairy legs will absolutely be subject to comments about how un-ladylike you are and that you look "masculine," even though body hair is entirely natural.
But it's not just when you're at the beach, and it's not just leg hair - it's everywhere, about everything, and it starts very early on. It's your parents telling you, advertisements and Hollywood reinforcing it, being told by your teachers you should be more feminine, your pastor saying that your outfit isn't "appropriate" for Sunday school (because it's not a dress), random men saying you should lose some weight... it never ends.
So while men are definitely having more body issues these days, it's nowhere near the same extent. Social media helps to reinforce societal beauty standards that have been taught to us all our lives.
> If so, do you believe this relies on a psychological tendency which existed prior to social media? If so, do you believe there's any evolutionary component to it, or do you believe there's some other explanation?
Not any innate tendency but just being beaten down until you conform. The advent of social media didn't magically erase the expectations we place on women, instead it became another place for reinforcing them.
> Women could still be quite invested their sexual prospects shortly before they hit their peak fertility.
I am single and have no desire to have kids despite being in "peak fertility." I am also not actively seeking out a partner. I'm not opposed to dating, it's just more convenient for me not to at the moment.
This is entirely anecdotal, but of my friends who do want kids, they don't really think about "oh god gotta find a mate and have kids I'm about to turn 26." They're more interested in finding the right person first, especially since we have so many fertility treatment options today (and having kids has gotten more expensive than ever). Having kids when you're not ready is generally a bad idea, and I hope I don't have to explain why.