Bibtex Key definition

Have you tried {{Full Company Name Ltd}}? In this way, Full Company Name Ltd is understood as the name. In the other ways, in the first Ltd is understood as the family name and Full Company Name as the given and mid names. For the comma, the format of names in BibTeX is Family, Given, so in that case Full Company Name is the family name and Ltd is the given name.

According to the BibTeX manual texdoc bibtex for MANUAL or PROCEEDINGS one can use organization instead of author. In such cases the field key takes precedence over organization. Thus you can try with

@misc{webpage1,
   key = {CompanyName},
   organization = {Company Name, Ltd.},
   title="The company's webplatform",
   howpublished = "\url{http://company.com}",
   lastchecked = {01.01.2013},
   year = 2013
}