My question is about SEO. How post or page of facebook, twitter, forum or classified sites(HIGH PR) show instantally in google or other search engines. As i know google take few days for crawl. Then how these sites's post or page show instantally. is it controll through sitemap.xml or something else please tell in in some detail. how can i do this for my websites.
The rate at which websites are crawled is decided algorithmically based on the frequncy content changes and the quality and uniqueness of the content. The more frequently your site has updated unique content the faster your site will be crawled by Google.
Because, Google Bots are getting smarter day by day. Millions of people are doing search on Google. For few of the search, Google will not have answer. It will keep those Keywords aside and as soon as you have any page containing those keywords, Google will map it and start showing it on the search result. If your page has meaningful content, your bounce rate will be low. So, Google will start liking you. Now say all your pages have quality content and with low bounce rate, Google bot will start indexing your pages much faster.
If you have a high enough reputation site you can ping Google when there are new articles. WordPress has functionality built in for doing so.
http://pingomatic.com/ is a ping service that will ping all the search engines when you ping its single service.
I've also heard that Google monitors the Twitter stream. Posting a link to a new article an Twitter will often get the page crawled immediately.
Google usually shows results from news blogs minutes after they are published. This requires a Google News verification as well as AMP feature enabled. Google also has a list of publishers who get frequently listed in the AMP carousel quite often.