Yes, people who:

A) don't want to be everybody's fb friends/g+/etc who were in a friend's birthday party, but still wants to share the photos she took at the party

B) easy for anybody to share an album and other to participate - usually after such a birthday as mentioned above, one person's photos are in fb, other one's in G+, third's in dropbox... in other words, people who wants to share together.

So basically everyone who has a need to share photos but they don't have a common service to do so.

(edited the format)