I intended to suggest this when the March update released, but it slipped my mind to post it until I saw Joscelin's request for an icon for map placement. http://bit.ly/psftpmapicon
I was hoping for an icon for the number of comments on an upload as well.
This would be similar to how Tumblr tells you how many 'Notes' a post has. (Notes in Tumblr's terminology is a cumulative count of both favorites and reblogs. Commenting on a Tumblr post is only possible when reblogging, which I'm not advocating for Photosynth.)
Likewise, Facebook and Twitter also show how many people have shared a particular piece of media, in addition to liking or favoriting it and the number of comments is usually easily discernable, either by a count (in Facebook's case) or by expanding the tweet or going to the tweet's own URL (in Twitter's case).
All three counts are useful information: Favorites, Shares, and Comments and can help persuade people to take a look.