I noticed within myself when I became actively poly, versus knowing I was poly and not doing much about it, that I wanted to tell the world about my new relationship and how magnificent it all was, and how everyone was getting along so well.
I think most people go through this, I've certainly watched monogamous people do this with their latest partner, so the fact that polyamorous people want to do it too should come as no great surprise. I suspect part of it has to do with the rush of new relationship energy, and wanting to tell the whole world how lucky you are that you found X (and Y and Z perhaps).
However, telling the world when you're poly is trickier than when you are monogamous. In my experience, this has stemmed from having to explain what polyamory is in the first place and dealing with various people's prejudices about monogamy being the preferred option, whether that be helped religious beliefs or their own ideals.
My own experience pointed to different phases in telling the world that I am poly. The first phase was telling people and then explaining it to them. During this period I was afraid of rejection from friends and family and went out of my way to explain that James and I were happily married, that our respective partners were new and sustainable relationships and that it wasn't all going to end in tears.
The second phase had me explain poly, but in a much more positive mind set. I had moved to the, "if you don't like it, that's your problem and not mine" mindset. Which meant that I wasn't afraid of rejection, because that was not my problem, and if someone walked away because of my lifestyle, then perhaps they weren't worth keeping.
The final phase has me now just expecting people to cope with the fact that I am poly. I will just drop into conversation that I have two husbands and a girl friend and expect people to keep up. This is actually the easiest, for me, of all the phases. I believe that because I have an expectation that people will keep up, and most of the hard work (my family and close friends) have already been told. Therefore, if someone objects now, then its not going to be as painful.
To get to the final phase I believe requires a level of confidence about yourself and your lifestyle choices. The final phase also requires you to not take rejection personally. To recognise that their rejection is their issue and if they cannot or will not cope with your choices in life, then that doesn't mean that you are not ok, it just means that they cannot cope.
My final bit of advice in relation to telling people that you are poly is to choose your battles wisely. We still have friends we haven't told because it isn't worth the angst it would cause. Not to us, but to them, and we don't want to upset them that much. Consideration as to how your employer might react is also worthwhile before outing yourself at work. I'm a Federal Public Servant in Australia, my employer can't discriminate against me, and my colleagues don't care. But some employers might, and since some discrimination isn't illegal, its best to be careful and know how your employer will deal with such things before telling them.
Good luck in telling everyone you want to tell.