1ef7220f__Guide___Salesforce_Developers__Details
[Field] LastModifiedDate [Details] 
Type dateTime Properties Defaulted on create, Filter, Sort Description Date and time when this record was last modified by a user. This is a standard system field. When a feed item is created, LastModifiedDate is the same as CreatedDate. If a FeedComment is inserted on that feed item, then LastModifiedDate becomes the CreatedDate for that FeedComment. Deleting the FeedComment does not change the LastModifiedDate. Ordering by LastModifiedDate DESC sorts the feed by both the most recent feed item or comment.
UserFeed | SOAP API Developer's Guide | Salesforce Developers
Details