‘Stay in touch’ means to keep contact.

I will be leaving today to go camping and will be away for a month, but don’t worry I will stay in touch.

‘Drop me a line’ means to send someone a note or a message in an informal way.

'As soon as you settle in to your new house, drop me a line.'

‘A shoulder to cry on' is someone who is willing to listen to your problems and offer advice.

She needed a shoulder to cry on and went to visit a close friend.

Flickr CC: Kevin Dooley