Hello.

1) try to comment user1's video
2) user2 comment user1's video
3) user3 comment user1's video
Result: you will get the notification "User2 and User3 posted a comment on user1's video"

APP_USER_VIDEOS_NOTIFICATIONS_COMMENTED_ON_USERS_VIDEO="%1$s posted a comment on {b}%2$s's{/b} video, {b}%3$s{/b}"

I beleave it should be _PLURAL and SINGULAR variations when somebody comment uanother user's item (not your).

The same for albums:

APP_USER_PHOTOS_NOTIFICATIONS_COMMENTED_ON_USERS_PHOTO_ALBUM="%1$s posted a comment on {b}%2$s's{/b} album, {b}%3$s{/b}"

& other stuff (photos, links... / in cluster too).

Thanks.