AFAIK there is no webapp that can accomplish such a thing...
The closest thing you'll be able to find is a webapp unit testing suite like Selenium. Basically, it's an IDE add-on for Firefox that allows you to record actions and save them as a script so they can be executed later. You can also write/modify scripts to do more complex tasks like your describing.
To post a specific message you'd have to write up some code that taps into the Selenium API to make it more intelligent. Information on how is probably more appropriate for Stack Overflow.
To do it remotely, you'd have to have a selenium server setup. Meaning that, if you want to do it 24/7 you'll need to have a dedicated server running a selenium daemon to handle the selenium tasks around the clock.
The reason you probably don't see any webapps online that can do this is the abuse factor. What your question describes is the essence of a spambot. A site that would provide it's users with the ability to auto-spam the net would receive a lot of heat for it. People hate spambots, many that are located are blocked, attacked, or even shut down by governmental agencies.
Sites like eBay, and Twitter are an exception to this because they encourage outside access to their site by using an external API. Sites like eBay Sniper (eBay), and Cliqset (Twitter) just use the tools provided with the API's to provide a richer user experience.