Invitation only is a reasonably successful alternative for niche communities, especially with the ability to banish an invite "tree".
My conspiracy theory: Campaign money, from the last few elections (I think "Correct the record" [1] was the first "disclosed" push), resulted in a bunch of bot accounts being made/bought all across social media. These are being lightly used to maintained some reasonably realistic usage statistics, and are "activated" to respond to key political topics/times. This is on top of spam accounts to push products and, of course, the probably higher-than-average bot number of accounts, made for fun, by HN users.