They think you are paranoid because they think that you are worried about Mark Zuckerburg logging in to your Google account or something along those lines. Explain that websites get compromised all the time - you could bring up the LinkedIn (http://lifehacker.com/5916177/65-million-linkedin-accounts-m...) or the Gawker (https://gawker.com/5712615/commenting-accounts-compromised-+...) compromise if they use one of those sites, and that when criminals get things like your Google passwords, they will often delete your data and try to scam your friends out of money - there are many stories, here is one about it: http://bits.blogs.nytimes.com/2007/11/09/e-mail-scammers-ask...