Then your problem is that you don't have a business model (much less a validated one), not that your traffic is falling. Even if your traffic is growing as before, you need to be similarly worried.
Get your business model figured out as quickly as possible, and cut your expenses like hell in the mean time. Get cash coming in from other sources at the moment (does not need to be from your startup). Hell, slap on some AdSense on your 200,000 uniques/mth site.
Honestly, quitting and going back to a job because of this little "setback" that still nonetheless leaves you at 167,000 uniques per month is pretty cowardly.
I don't know what you mean by "media sharing," but you're going to have to figure out where money can come from. It doesn't have to be "a business model," just something to sell. Eyeballs, accounts...whatever.
Then you don't have a startup. All you have is some website.
Figure out how to make money from it, or don't waste your time. If it breaks even, you can just keep it running, but from my experience, sites require maintenance, and, depending on how much you make per hour, it can be pretty expensive to support something that doesn't pay back.
It pays for itself, but doesn't make me any money. =/