If I found my job posted online or wherever I'd probably already know about it.
But speaking as someone who recruits and trains people for a living, constantly recruiting in a company is a very good way to seperate the wheat from the chaff.
Depending on what you do, try and prove your worth, step up to the plate and watch everything you do.
Any firm (that wants to preserve itself) needs a reason to fire you, so don't give them one.
And if you're sure you already have, start making inroads to a new job while still doing the best you can there.
But don't whatever you do just throw your keys in without letting the situation. The reason for this ad may not be aimed at you. Company expansion, giving you assistance, promotion?
Its not the end of the world. yet.