Any site with 3000+ members is not private.
Any site that you can read more than the first post in a thread without logging in is definitely not private.
Safe, in internet terms, is a site that requires HTTPS: (Secure HTML) addresses for every page, and/or requires a VPN client login before access. My work has a VPN (Virtual Private Network); I can’t access anything without using the secure VPN client, and even then, some of the pages use HTTPS: calls for added security… then again, those pages deal with student records, employee records, and other such data. The most secure of those can’t even be accessed outside the hard private network.
Secure is purely relative. HTTPS calls can range from negligible (as low as 24bit encryption) to pretty darned good (1024bit or more, single session keys). VPN’s can be likewise weak to strong. Plus enforcement of password standards can range from lax to draconian, and pasword minimums from trivial to insanely tough.
No system on a network is “truly secure” from other machines on that network. True security is a HPN that has no connection to other networks.