As is and has long been the case at most carriers, DL does seem to view partner awards as a way to clear out inventory without discounting the cash price (since fairly few people interested in flying domestic are going to have miles in other SkyTeam programs or think to search there).

​​​​​So, you'll see them open availability to FB when they don't think such a sale will keep them from getting a late cash sale, which suggests to look fairly close-in and probably at one-ways. For instance:

BOS-SLC-SFO isn't that much shorter than something like BOS-DUB...

Searching for cash fares on DL, most BOS-SFO (nonstop and connecting) are north of $1k, though these particular flights are bookable for about $630 incl. taxes (so less than 1 cpp).