Are all of you on the same ISP, maybe Metrocast?
If you are then this may be a DNS problem. DNS is the service that translates the domain name to an IP number. In future posts please include your ISP so we can see if there is a pattern.
One way to get around your ISP's DNS is to use
OpenDNS. OpenDNS is a free service that is far better than most ISP's DNS. Many large companies, schools, etc. have switched to OpenDNS. I use it at home and at work and I've never had a problem reaching these sites. You don't have to sign up, just use their servers.
Their DNS servers are 208.67.222.222 and 208.67.220.220. You can enter these settings fairly easily but the procedure varies for different kinds of computers and OS versions. Just Google "how to change DNS in windows" (or Mac) and you'll find plenty of help.
I can't say for sure that this is the problem but I've seen DNS cause these kinds of problems in the past.