Comments are an enduringly popular thing in social networking (even, it turns out, in some forms of social networking that don't seem to have them on first appearance). Ostensibly one reason for this is the power of feedback, but that's not a complete answer; as some people have noted you can have feedback without comments as such, especially if you're doing it within a closed system.

I've recently had a realization about a core reason why comments are good within a closed social network: they reduce update volume.

First, let me take a step back. I've wound up thinking that there's generally only so much update volume a given site can support, both for interface reasons and because most people only have so much time for your social network. You want to get as much content in front of people as possible, but you want that content to be interesting content. Uninteresting content invites people to scale back (ie, walk away from your site) just as too much content does.

If your social network is successful, people are going to have conversations with each other somehow (either in comments or in regular entries that go back and forth). This is a good thing, sort of; conversations create volume, but not all conversations are interesting to everyone. Using comments for conversations creates a second class sort of update that avoids bombarding uninterested bystanders with volume. There can be a massive conversation in the comments to some entry and you'll barely notice from the outside.

(The social network would like you to get involved in the conversation, but only if you're going to be genuinely interested. Hence the logic of showing you some recent comments, as places like Facebook and Google Plus do.)

Of course, you don't need comments per se to do this. What you need is some general aggregation mechanism that groups the conversation together and hides most of it; with sufficient cleverness you could do that with threaded entries or something similar. Comments are just the mechanism that everyone understands.