But you'd think they'd do something like '200 users have liked...' A cron like every 5 minutes (or more) after the first 10 likes or something like that..

The problem with these systems is that batching would make the implementation more complex. Sending say 10m notifications per second in a fire-and-forget manner is easily distributable en requires little state. Batching per user would be do-able but requires thinking about distribution (messages to the same user should at some point go over the same machine) and tracking a bit of in-memory state per user.

In short I can imagine the trade-off of less-optimal support for some edge cases like these.

Yeah but then is it really notification worthy? If you're that popular, you just post, and then check back in a while to see how it did. There's nothing really "notification worthy" like when it's more like 1-to-1 communication.