For one thing you can use VC money to pay for ads

But for a paid social network you can do that too. All you need is for CLV to be higher than CAC.

If you want to launch a big viral marketing campaign, you also need someone who's willing to trade your predictions of future revenue for cash today, and VCs are by definition the people interested in making that trade. For something that depends on network effects, slowly trickling out one new ad every time you get 10 new signups isn't going to work.