Here lately, users have been necroing extremely old and long dead topics. This means they take a topic that is 3 months old and respond to it. This is generally bad forum etiquette.
There are, however, exceptions. If, say, you have a thread titled "What's Your Favorite Video Game?", it's acceptable to post a reply on the thread, as creating a new topic would simply be a repost.
However, if you encounter a thread titled, for example, "Katisacat's Photo Album", that has not had a response in, say, 3 months, please refrain from posting. This also applies to old, long dead role plays, news about old events, etc. Use your common sense when looking at a thread. Will your response really contribute to the thread? Is your response relevant any more? If the answer is no, don't post.
Thank you. I will be amending the forum rules later on concerning this topic, so keep an eye out.