Sort merged array by array value in Twig?
The SuperSort Plugin is built for use cases exactly such as this. It allows you to render each object in your array using a Twig object template and sort the array based on the rendered value.

For example, in OP's case, we could sort the combined array by the created_time variable like this:

...
{% set feed = instagram|merge(community) %}
{% set sortedFeed = feed | supersort('sortAs', '{created_time}') %}


(The plugin parses {created_time} &mdash; a Craft 'micro-template' &mdash; into {{object.created_time}}, which provides the value by which the array is sorted.)

If the property of interest happened to be different between two arrays, we could include a conditional or ternary operator in the micro-template to differentiate them:

{% set sortedFeed = feed | supersort('sortAs', '{{ object.current_time is defined ?: object.someOtherProperty }}') %}