As someone who is terrible at keeping in touch and generally hates social situations, I enjoy the passive aggressive option of 'liking' a post from someone I haven't spoken to in ages. Just to remind them that I still exist, not that they'd care anyway :C
A 'like' on Facebook is similar to going to the local chip shop and waving to a friend passing by on the other side of the road while you wait for the young lady serving you to finish with your saveloy.