I think this is shortsighted. The lack of negative rating is an issue. Even though social networks do not allow for them they should at least try to infer them as a 'hidden' or 'latent' variable (from the lack of sufficient amounts of up-votes after a large amount of views).

I'm pretty sure that G+ doesn't try to infer downvotes because of the large amount of sensationalism, alternative medicine, celebrity gossip in the "What's Hot" circle present in your feed by default. These type of posts would not rise very far on Reddit.

Also, downvotes are important when it comes to computing trust metrics in recommendation engines or buyer/seller rating frameworks especially if you want to be able to use transitive trust metrics (A trusts B and B trusts C therefore A can trust C). Negative trust ratings are useful in avoiding bad products or bad actors. The availability of these ratings would be quite valuable in helping with online transactions and give confidence to users the first time they buy from a new source.

Without the two data points, it is more difficult to determine the parameters of multinomial/dirichlet distributions used in, for example, subjective logic which can be used to build networks of trust. see http://en.wikipedia.org/wiki/Subjective_logic