We are aware of the warnings generated by PHP 7.3. Please, take into account that PHP 7.3 has been released as stable in January, and Joomla core is also fixing these warnings now:
About FOF 2, it works perfectly fine. In our plans, we are in the process of implementing a new architecture for AutoTweet / Joocial v9. In the meantime, if we confirm a compatibility issue, we'll solve it in the extension.