There is the personal "front page"(stuff you subscribe to). Then there is the public "front page" which is just the most popular posts on reddit sans some controversial or NSFW subreddits.

Anyway, here's the frontpage of r/popular for me, is the "entire frontpage" full of advertisements by megacorps? Looks like I might be missing something, can someone tell me why each of the, say first five posts, are actually ads?

https://i.imgur.com/4x3CugK.png

Not saying this is the case in your example, but take the Mariah Carey post: it wouldn't be hard for someone to buy up legit-looking accounts and make a bunch of posts about Mariah Carey (or whatever person/thing) to refresh everyone's memory about the topic in advance of some new concert or movie in a few months.

Again, it's not necessarily the case in your screenshot, but at the same time, how do we tell for sure?

Yeah. Reddit are gearing up for an IPO about to launch soon. They need to convince investors of their ability to push ads. Reddit ads most of the time just look like posts. Sometimes (top) posts themselves are cleverly disguised ads. In short, they are just about to go full corporate.

Exactly what effective propaganda would have you think!

r/popular also defaults to a different page by region, so again, I won't see the same posts as the GP.