Are private proxies safe for facebook/twitter? i mean ... they will be in the same C range with others spammers, even if they are "private"

I wouldn't trust 100%. Eg never login to your bank.

No. I mean for bots only...

the problem is that i think most proxies are in the same C range. So it will be very easy for facebook/twitter to trace back and make a mass ban.

Proxies are likely shared between multiple users. if someone else from that IP is using it, it'll likely end up getting blocked in the long run.