Should I from now on always note when a post is a joke?
Might be a prudent thing to do. With written communications, there is a fine line between sarcasm and stupidity. A lot of posters include a "/s" at the end to indicate that the post is not to be taken literally but considered sarcastically.
There was nothing in your post that indicated any sarcasm. You might want to keep that in mind before accusing someone else of not having a sense of humour. A wise man looks toward himself before blaming others.
If this is not the saddest thing you have seen in a while, why did you entitle your post "the saddest thing I've seen in a while"?
Perhaps a better and more accurate title could have been along the lines of "here is something I found amusing/strange". That would have prevented any misunderstanding.
Just a suggestion.