to have regular communication with someone by telephone, letter etc..

Are you still in touch with Caroline? (usually + with)

• flirt with
• flirt with something
• float an idea
• flog a dead horse
• flog something to death
• flog to death
• floor it
• flotsam and jetsam
• flunk out
• flunk out of school
• flush out
• flush out someone
• flush out something
• fly blind
• fly-by-night
• fly by the seat of pants
• fly by the seat of your pants
• flying high
• fly in the face of
• fly in the face of something
• fly in the ointment
• fly into a rage
• fly off the handle
• fly-on-the-wall
• fly on the wall