With former co-workers who were never more than 'work friends' and ex-girlfriends it would be a tad creepy to try and 'friend' them when you really don't want a conversation with them when really just an anonymous update would do.
What made me think of all of this was seeing a neighbor walking their pet greyhound down the block– you see I used to work with a woman who spent most of her week-ends providing for retired racers who were sent out to pasture, in effect. She had a few of her own and was always looking for other people who could adopt these poor animals. She could talk to you all day about them, you really could say for her it was a passion of hers. Always knew which car in the lot was her's, too as it was covered with dog stickers and a greyhound license plate as well. I thnk she loved those racing dogs more than her own kids some times!
Always liked the look, never had the home for one but glad to see one in our neighborhood.