It is considered acceptable to tag a post once when contributing to a specific conversation.
Two hashtags are considered acceptable when adding a location to the conversation.
A hashtag must begin with a hash character followed by other characters, and is terminated by a space, or end of message.
It is always safe to precede the “#” with a space, and to include letters without diacritics, digits, and underscores.
In order for topics to trend, there has to be a consensus, whether silent or stated, that the hashtag refers to that specific topic.