Same happens for me with HubPages and with the University! What it means is that those companies do not buy security certificates, so the virus checker you are using does not "recognise" them as having a security certificate from a recognised provider. If I am going onto a big site and one that I use frequently, then I just ignore the warning. If I am being diverted to a site I do not know, that may be malicious, usually the warning is different and it tells me that the site may be malicioius. In that case, i stay away from it.