Apple iPhone owners please read.
Do you have friends who previously had iPhones and switched to Android, who now ignore all of your texts?
Well maybe you aren't being ignored. Apple has a setting that will not send messages to Android numbers if the connections were previously established via iMessage.
To fix this:
Go to > Settings > Messages > and turn on the feature "Send as SMS" (Send as SMS when iMessage is unavailable.).
If this is working properly, your SMS texts to Android users should appear green. iMessage messages are blue.