Information gets old. Cheese rots. Membership to a website is over. There are many things that expire over time. The main reason things expire is because information is updated, someone wants more money, or food just went bad.
There is no set expiration date for a blog. With daily posts containing updated information, the blog itself will never get old. There are some blog posts that may have false statistics such as “Twitter only having 100 million users.” That was true a while ago, but now that statement is false. Twitter currently has over 500 million users, but that can change as well.
New information contradicts old information, more money is needed to regain membership to a website, and cheese rots so you know if it is safe to eat or not. The visitor is always searching for something. Make sure you give them something without an expiration date.
The only way to do that is to constantly update your blog, Twitter account, and everywhere else that you write online.