in my search area, I've noticed a substantial drop off in "new" SBs.  It went from around 40-50 per day down to less than 20.  I do not use any filter except for distance and that hasn't changed.  Is it because I don't specify anything under the "what I am looking for" tags?  

If you are in a college town it could be that they are going home for winter break. Or they just flunked out! Lol

As SFtD suggests, the newbie sign up rate can vary based on several factors like school schedules, holidays, how close they are to rent or car payments being due, and more.   Even in Los Angeles, I see periods of two to three weeks with little or no new profiles showing up. Then I might see 10-30 in a single day.  I keep my filters pretty tight, so there are probably many more that what I see.  
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My typical filters:
- Distance:  10 miles (25 if I don't see enough hits)  
- Age:  18-28 (sometimes I'll go up to 36 if I don't see enough hits)
- Body: Slim, Athletic, (and sometimes Average)
- Non-smoker
- No Children
- Lifestyle:  Minimal, Negotiable (and sometimes practical)

I have found it's better for me to include "curvy" as well because a lot of women overestimate how fat they are!  I've seen curvy's that really should be slim, maaaybe average.  

Whoa, really?  I get all of these “thin” or “athletic” girls who are anything but that. Wish there was a skinny fat category for all of the girls who have no muscle tone also.

Gotta agree with that, curvy to them can mean big breasts on a regular body.

Yup, sometimes the newbies don't realize that in the Sugar Lexicon "Curvy" is a euphemism for chubby. So every once in a while I do a search for only Curvy, just to see if I've missed some, and often I do find a few!  Actually, my current favorite SB who has a super fit, slim body listed curvy in her original profile.  When I asked her about it she said but I am curvy!  Truly she is, but it's all the right curves in all the right places and no extra fat.  

You went from 30-40 newbs per day down to under 20?  What sort of Sugar paradise are you living in?  I'm lucky to see 5 new ones on any given day and luckier still if one or two are actually appealing.  Wow!

I screen here in SWFL for 50 miles, slim, athletic, average and curvy...a typical week here produces only on average about 5 or so newbies !
You guys near college towns are in SB Paradise 😋

That's because I have my distance filter set to a hundred miles (which can be up to 2 1/2 hours) to cover Charleston.  Only about 7 or 8 of those are actually within an hour of me.