Two worldviews on this. You could say "Twitter is an ad company that happens to sell ads on a social microblogging service" or "Twitter is a social microbloggin

Two worldviews on this. You could say "Twitter is an ad company that happens to sell ads on a social microblogging service" or "Twitter is a social microblogging service that makes revenue by selling ads". Same with Google. Some call it an advertising company, some call is a search/services company.