If there were really as many civilisations around as some say, we'd have been in a dozen radio shells already and would know them. Things are always more boring in real life than they are in sci fi.
So we have to consider that, either no civilisations exist close enough for radio waves to ever reach us, or that they either disappeared too early or appeared too late for their radio shells to be detectable. Or that they never developed radio at all and don't want to.
Personally I'm not a fan of alien civilisation ideas, since they don't usually take into account the vast size of the universe and the vast number of difficulties and incredibly unlikely factors faced with not just with a star system being suitable for life, but suitable for long-term life and a civilisation. It's a near possibility. However, the universe is rather large, so near impossibilities would quickly become certainties...
I'd put the figure at maybe an alien civilisation every 12 galaxies, at a pessimistic guess. Which means we'll never, ever contact them using simple electromagnetic transmissions. Unless there is a more instantaneous way, they may not exist at all because we'll never detect each other. Which may actually be for the best. Evolving to survive and rule one planet was hard enough, how difficult would competing with 1000s of alien life forms be? :-P
