Time taken by VACUUM FULL to reclaim space
It's a busy server of a local social networking website. No time is actually &quot;downtime&quot;.

So VACUUM FULL is going to be a problem since it takes an exclusive lock on each table it processes. Consider the community tool pg_repack instead which achieves the same without exclusive locks.
Related:

VACUUM returning disk space to operating system

None of this affects the size of backups, since those do not include dead rows to begin with.