2) Should I use a VPN in addition to TOR even if I am not into illegal activities (but deep web social networking)?
Yes. Ideally, you would have a paid VPN. Personally I quite like TorGuard, but there are
many others. If you are not doing anything illegal though free VPNs can be okay, because you wouldn't need to be worried if they give logs to the authorities.
1) What settings should I use in TOR browser?
-You should try using the operating system Tails.
-Disable scripts to stop sites trying to track you.
-Keep your Tor window in the default size rather than enlarging it.
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Consider using software to help get rid of cookies. Personally I don't bother with this part.
-Use a decent search engine, like Tor's default engine DuckDuckGo.
-Ideally, use Tor in a variety of locations (not essential).
3) Any additional security advice?
You should consider running your coins through a mixer before sending them to the site.
Also, please PM me the URL. I'm interested in social networking on Tor.