Comment by megolodan

Hmm consider an established social network which sees lots of bad actor activity, these folks will likely be sharing IP's on these residential network severly damaging their reputation.

You should only see one user-id per MAC / IP. If you see multiple then its a sign of a bad actor.

Before you're established using something like a captcha prevents most spam, except for state actors, and they wont be focused on the site until its larger.