That’s a really tough question, which I’m unsure if there’s a correct answer, in other forms of market segmentation, there are boundaries limited by media form or by the channel  market etc. but the net is really too broad to gain an exact answer because it’s quite subjective.

If you’re looking more at the competitive environment you could use the moz Keyword Difficulty & SERP Analysis  to try and understand rankings  vs authority + competition  but be mindful that the long tail has many more variants that make it tough to get an exact answer, in the past I’ve looked at syfu  to gain an understanding of paid market share based on spend but  the limitations are on account structure.

You can look at adwords to gain an understanding of local/ global impressions vs rankings  to understand impressions share.

For social Joanne Lord did a great video via distilled about social, I think it was on last month’s free video list.

You can only make an educated guess by pulling as much competitive data from as many sources as you can.  Sorry if that’s a bit broad!