I have been asked this question enough times that it warrants a post. When an agency downsizes, is the last person hired always the first person let go? A lot of juniors worry about this. At that level, they really shouldn't, but everyone knows Advertising has the reputation (rightly so) of being unstable and so they ask. Accounts jump often enough that job security in the Ad industry is an oxymoron.
From my experience in letting people go, the last hired is not usually an indicator of who gets cut. Many, many factors go into this process. And, yes, it is a long, painfully hard process to decide on who is let go when the economy forces a downsize. To start, we look at skill set (what you know how to do), how much money you make (to have the biggest financial impact with the least amount of people), and how versatile you are on other accounts or in other mediums. We may consider your hire date, but certainly not exclusively.
If you bring some great creative chops to the table, if you are making an entry level salary and are experienced in more than one medium (say digital and print), chances are you'd be one of the last folks I'd consider letting go.