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Is Google allowed to do that? I mean, can they go beyond showing a meta description on SERP?
You can't copyright facts, so if your site deals in that, expect to be pushed out. However, it gets extremely risky for Google if they're going beyond that.
...fair use is a doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders. Examples of fair use include commentary, search engines, criticism, news reporting, research, teaching, library archiving and scholarship. It provides for the legal, unlicensed citation or incorporation of copyrighted material in another author's work...
It is important to remember that even if a database or compilation is arranged with sufficient originality to qualify for copyright protection, the facts and data within that database are still in the public domain. Anyone can take those facts and reuse or republish them, as long as that person arranges them in a new way.
If Google can display a sentence of your content or even a paragraph and it fully satisfies the consumer causing you to go broke, then your business strategy needs rethinking.
what proportion can be reduced to two lines in a SERP or knowledge graph?
[edited by: JD_Toims at 12:33 am (utc) on Dec 20, 2013]
Lucy, these pages do not look like "height of Eifel tower" in that the knowledge graph content replaces the results side of the page with actual content from various sites(without titles/links/descriptions). You no longer see traditional results above the fold in the new layout.
maybe the Knowledge Graph only kicks in when there is a known, factual answer.
...For a search that seems to be about specific medical symptoms like "cough at night", our algorithms analyze the web search results to find health conditions that may be related to the symptoms in the query....
We've found that certain search terms are good indicators of flu activity. Google Flu Trends uses aggregated Google search data to estimate flu activity.
WebMD authorizes you to view or download a single copy of the material on the WebMD Site solely for your personal, noncommercial use if you include the following copyright notice: "Đ2013, WebMD, LLC. All rights reserved" and other copyright and proprietary rights notices that are contained in the Content.
All that said, if your business model depends click-throughs to retrieve publicly available information, I would rethink the business model.
What is public domain can include just about anything viewable to end users and should not trump the rights of the creators of such content.
if your business model depends [on] click-throughs to retrieve publicly available information, I would rethink the business model.