I am looking for some source (fb, twitter, website etc) where is posted from time to time cheap offers of flights (mostly interested from Germany). For example there is a cool fb page called "mleczne podróze" for Poland.
There is a good collection of sites, however be aware that these are mostly in German, so Google translate might be useful. Here are some examples but there's more. At least travel-dealz is somewhat transparent about their sources so check where they get their inspiration from and follow these as well. Also they typically publish vacation/hotel deals as well that you might be less interested in. At least Urlaubspiraten has a good filter though (I'm already linking to flight only deals below).
They take much of their inspiration from forums such as Vielfliegertreff (in German, the international equivalent is Flyertalk) where cheap/error fares are typically discussed first (if you want to become active be aware the tone is somewhat rough).
There's also a few similar websites with an European focus that might be as useful to follow. My favorites are
If you are looking for resources on twitter, this luxury travel blog has a very decent list of twitter accounts to follow. (The guy is Austrian, the website in German and his travels / the list international, so pick what you find useful).
Needless to say, sometimes there's many good deals and sometimes just nothing, you have to be patient and ready to book when time comes around. Good luck!