It has recently emerged that the German Finanzamt has been using social media sites as a source of information for ongoing investigations, looking keenly at individuals’ status, occupation and location for information that may deviate from what has been reported on tax returns and residency statements. Often, discrepancies between the reported and the 'friends-only' information published on social sites can be a useful tool for the authorities if suspicions have been aroused.
This really shouldn't come as a surprise. In Europe it's not unusual for the authorities to contact landlords to confirm whether an individual truly lives at their stated address and in the UK sources such as phone records and travel tickets are routinely used in residency cases. The use of this new resource should simply be seen as an extension of this practice; any available means will be used to verify the information provided by taxpayers and challenge those who decide not to be truthful in their dealings with the tax authority.
This also serves as a reminder that the default settings on most social media websites are very open; if you are not willing to share any aspect of what you have published with people you don't know you should look to go through your settings and make sure that these match your expectations.
Tax rules, interpretations and priorities change from country to country. Before you make a move to a foreign country, you should always seek professional advice from a local expert and ensure that your strategy makes sense in light of local legislation.