I suppose it just depends on the couples in question. Some people are just overly more affectionate or just post it on their Facebooks to show off the fact that they have a partner ( i can't count how many times i've seen people manipulate their partners to make their exes who are usually my friends jealous).

Personally, i value my privacy and as such i don't feel the need to tell everyone about how long a relationship with my partner has lasted for. If i did decide to celebrate it however, i would probably just have a quiet dinner with my girlfriend or take her out somewhere nice. No need to let the whole world know.