Looks like Pixelfed's growth spurt is slowing down. Absent any new external stimuli I'm guessing it'll stabilize around 200K to 300K monthly active users -- over a hundredfold order of magnitude from what it was just a month ago.
Yes a fediverse version of Instagram. It relies on the activity pub protocol under the hood. Which makes it possible for example to follow like and comment to a pixelfed account from a mastodon account if you have one.