They aren't really networking sites, they are more databases of the direct contact information for managers, agents, and publicists. I feel like these sites are gems that a lot of people don't know about.
I use Bookingagentinfo.com for getting my beats directly to major
artists. I just e-mail their managers a little package of beats that I created specifically for their
artists. Though not every manager responds, but I've had about 5% of them reach out to me telling me they like my style and to keep sending them stuff. I think I've had success because I don't dont spam them, and I contact them with their
artist in mind. I also use the site for my clothing brand to get some rappers to rep my brand, I've had even more success with that. The site pretty much has every
artist that I need, and if
an artist isn't in the database, they'll add into the database within a week.
Contactanycelebrity is good too, and has a bigger database but was more expensive so I ended up canceling my account there.