Google Webmasters's dashboard gives you a big number which is called impressions, and by definition that I've seen in Google Analytics, it means the total number of times your site has been become eligible for SERPs.

I just don't have an idea how to invest on this number, and how much its increase or decrease mean to me, because I can't compare it with other websites.

I mean, if the impressions of say site a.com is 150,000, and mine is 50,000, then maybe I can confer that I need to triple my efforts to reach to a.com. But by seeing 50,000 alone I have no clue at all of how to interpret it.

Is there any service or other way to know about the impressions Google gives to other sites?