V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

TTFN
       Ta-Ta For Now (slang, Usenet, IRC)


The Free On-line Dictionary of Computing (30 December 2018):

TTFN

    ta-ta for now - goodbye for now.  Used in the UK, USA
   and probably elsewhere.

   (1998-07-02)