trademarkFor instance Facebook is a trademark.
If I start another social network tomorrow, Is it legal if i use description like "In a nutshell we are another Facebook. But we have additional XYZ features"?
Yes, it’s OK as long as you make it clear that the trademark is not your own. You should include something like the following in the footer of your website, along with your own copyright and trademark statements:
“Facebook® is a trademark of Facebook, Inc.”
or
“All trademarks, service marks, trade names, product names and logos appearing on this site are the property of their respective owners, including Facebook®, Microsoft Windows®, …”
The question does extend more deeply than this, and there may be additional considerations if there is any ambiguity about describing the other party’s trademarked services.
Source: http://www.dmlp.org/legal-guide/using-trademarks-others
Quoting what I answered in this related question:
https://startups.stackexchange.com/questions/11160/can-i-use-other-company-name-in-my-advertisement
Generally speaking, using another company’s name without authorization is considered fair use but using its logo, characteristic elements of its visual identity (e.g. its typography), or implying its endorsement without consent is not. Put another way, don’t be misleading about your relationship with a third party. (Particularly when the third party is a large firm that can lawyer up.)
If you follow the first (inta.org) link you’ll find plenty of case law related to that for the US.
Aside: it’s not always a good idea to describe your business as the X for Y:
https://startups.stackexchange.com/questions/3327/is-it-good-to-describe-your-business-as-x-for-y/
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