Whatever you post, whether it be on your Facebook page, Twitter, a forum, or any internet site, that information is available to HMRC and they will look at these sources for information to check your tax return.
One of your obligations is to ensure that your tax return is accurate and complete. HMRC is alerted to various entries in your tax return such as rounded amounts or what HMRC suspects are estimated amounts as they will then believe that your record-keeping is not being complete.
If there are any year on year inconsistencies or variances in your tax returns, for example, if you have lived off non-taxable incomes one year or there's been a change in your business performance such as your gross profit rates, you should make entries in your additional information box, also known as the white box, to explain this. That way, when HMRC risk assesses your return, you will have made this situation aware to them.