The way I see it, all of social media is braggy. There is nothing not braggy about it.
"Look at this selfie of me being cute."
"Look at this pretty cake I made."
"Look at my cute baby."
"Look at this status I wrote."
"Look at my beautiful holiday decor."
"Look at me graduating with a degree."
We are just hyper focused and sensitive to TTC because that is where we are in life. As far as people talking about how quickly/unexpectedly they got pregnant, I think that if the opposite is okay to share (fertility struggles) then "easy" fertility journeys are okay, too. A lot of people have mentioned how much support they find from posters who talk about their TTC struggles, and I'm willing to bet that there is a whole group of women who are comforted that they aren't the only ones who had an "oopsie." You said it yourself that there is a stigma about them being "irresponsible." My sister is a young mom who had two unplanned pregnancies, and I know that she follows social media accounts of moms who are in the same boat.