Percentage of market share is not a clear indication of monopoly. From what I could find, 50% is typically the bare minimum to warrant consideration, and 70 - 75% will get more serious scrutiny. Also, it differs from country to country. In the UK, Tesco was investigated even though it had a market share of only 30%.

I also recall reading one of the EU's regulators (was it Joaquin Almunia?) say that (paraphrasing) "we start sniffing around when any one party gets a market share north of 60%." Unfortunately, I cannot find that article.