If you run your own Facebook business page or not you have probably seen the response time listed next to a business online "business usually replies within minutes" and think great ill drop them a message now and get an answer to a question I need to ask them.
Great for the end user, but I have heard some stories from business owners who are panicking when they see a a message come through to their page at 2am and they think they have to reply there and then. No you don't! I have seen other marketing people think you have to reply too, thinking you have to have the last word in a conversation.. relax you don't have to do that either.
Facebook is a clever tool, it knows where we are, who we're with and when we're online or not. It uses this to know how it calculates a Facebook page response time, if you're online and a message comes through the time starts ticking.. but if you aren't online or inactive such as being in a meeting then the clock doesn't start.
For example, if a message comes to your page at 2am, you wake up and login at 8am then reply to the message at 8.05am; your response time is 5 minutes. With regards to having the last word, each conversation is measured not each message. Conversations are measured in 24 hour periods so if someone replies with thank you, then you don't have to have the last word. The conversation will reset after 24 hours and if they message again after that then the clock will start ticking again.