Interesting, but I don't think Google wanted to go social to decrease the damages of link spamming.

People already do social network spamming. There are plenty of sites where you can pay for a certain number of +1's or likes.

Using a combination of inputs, social interaction, page links, keywords, they can achieve a better overall ranking algorithm, which is probably one reason they wanted to go social.

However I think the main reason Google wanted to go social was to be able categorise their users better. The more they know about their users, the better they can serve ads.