Tough Saturday. Frannie and I passed this one back and forth, which is rare these days. I did most of the top, she most of the bottom, and we both stared long and hard at the last two squares - the cross of NATAL (27A: Brazilian state capital) and LAMPED (28D: Clobbered, in British slang). LAMPED? Really? I call foul. And the other cross, which Frannie eventually got, was AKA (46A: Going by) and CAKEPOP (37D: Snackable treat on a stick). AKA is good, but if CAKEPOP is a thing, it's a thing I don't know.
Things I liked - TINFOILHAT (56A: Stereotypical wear for a crackpot theorist), CARAMELS (10D: Sticky treats), TERENCE (38D: Ancient playwright who specialized in New Comedy), NIGERIA (35D: Country of 180+ million people that has never participated in the Winter Olympics) (interesting), ATEASE (44D: Chilling), and TIGRIS (25D: River flowing from the Garden of Eden, in the Bible).
Things I didn't like RURALIST (34D: Definitely not a city slicker), MOMMAS (30D: May honorees, colloquially), MAGE (15A: Dungeons & Dragons class), the repetition of "up" in PRIEDUP and RARESUP, the backward cross-reference in 1D and 4D, and the two entries mentioned at the start.
1A: Facebook acquired it in 2014 for $19.3 billion (WHATSAPP) gets a C. It should have been my first answer, but I couldn't come up with the name immediately, so THEMARTIAN ended up being my first confident answer.
I guess I'm a little down on this one overall. I didn't particularly like the long diagonal of black squares that cut the grid in half, there were those dislikes, and I don't know... it just didn't do it for me. Not all bad, of course, but not my perfect Saturday puzzle.