As others have stated, feeling alone is pretty common with PTSD. Sometimes you can have many friends but still feel isolated and alone.
I tend to have different types of friends. Some are friends due to a common interest or activity we participate in. Others are friends due to similar experiences we have shared. Other are simply friends because we've known each other for so long, even if we don't share as much anymore.
There are many different ways to meet and make new friends. Check out local meet-ups; (there's a website, but I don't know what it is). This a great way to meet others who are also looking to meet new people and start friendships. Also, try taking a class, perhaps through a local adult education center. Not only is it a good way to help fight off the blues by getting out and doing something fun, but it also can be another outlet to meet others.
Good luck, and take care.
Nic