As Nelda's link indicates, comments on profiles can be deleted or archived by the person who added the comment. The profile manager also can archive a comment by a third party or delete it if it's a comment on their own profile. Deletion of a comment also deletes the replies to the comment.
Many of us add genealogical content in profile comments, but that's not what profile comments are for. Profile comments are designed/structured as a communication tool (for communication with other folks who are interested in the profile). They are not part of the profile. When we want assurance that our content will be available in the future, we should put that content in a person profile or a free-space profile. Remember that "Research Notes" are appropriate if we are unsure about whether the content is good.
