For example google.com can be tagged as a search engine. facebook as a social networking site
I want to automate this process. How do I go about it? That is, I want to create a tag engine which automatically labels websites.
I can pull up the meta information for each website and then guess the category.f there are sites already there on the net which does the same thing
As you said, you could easily extract the meta keywords and description from a rendered page. However, depending on your purpose, that may not be too useful. It's doubtful that major social networking sites actually list "social networking" as a meta keyword.
For automatic labeling with correct categories across domains, you could write the program to query a web directory like Google Directory or the Open Directory Project (ODP). You'd think there would be a public API for Google Directory, but I'm unable to find it.