One of the guiding principles for 'what site should I migrate this question to' is, 'keep it where it is, unless it'll get much more love somewhere else.' This very question was brought up many times back when the whole Area51 thing started out. In brief, The Powers That Be are not afraid of overlap. In fact, overlap can be good. Overlap builds (multiple) communities.
Specifically regarding the AskUbuntu and Unix/Linux point: https://meta.stackexchange.com/a/72753/131633
Where things get tricky, especially for a site like ours that has been here long enough we now have questions that are over 2 years old, is the shifting values of 'topical'. Thanks to the Ubuntu, and especially the Unix & Linux stackexchanges, we are no longer the default site for "how do I do complex things on a [generic] Linux system" questions. It has allowed us to focus more tightly on the first line of our FAQ:
Server Fault is for system administrators and desktop support professionals, people who manage or maintain computers in a professional capacity.
We are now giving the benefit of the doubt to ever fewer questions here. Unless it's clear that something is in a professional capacity, we're much more likely to shovel it on over to SuperUser or just OT-close it.
That said, we do have Pro-Sysadmins who are up to their eyeballs in Sharepoint or Wordpress. Their questions, if asked on SF, shouldn't be immediately OT-migrated to their respective SE sites unless we simply can't handle them. They are topical here. The same goes for Mongo, MySQL, and MSSQL. The key thing is "professional capacity".
As for Keeping Track of All The Things, one of the coping mechanisms they created was the ability to follow tag-sets on multiple sites.
http://blog.stackoverflow.com/2011/04/improved-tag-sets/
This would allow you to follow mongodbon SF, DBA, and even SO and SU.
