I agree that there is variation in what reasonable people can expect re: credit for creating something. I think if attribution is important to you (general), there's no harm in being more proactive about it--whether that's watermarking photos, posting them only at sites where they can't be downloaded easily, including a message on your FB page asking people to always credit you, or politely tracking down people who leave off the attribution and saying, "Hey, could you please tag me in the photo, since I took it? Photography is a serious hobby of mine and I like to know where my photos go, and make sure they have my name on them. Thanks!" No need to sit and stew about it, or assume malicious intent--I feel like most of the people I know are pretty lax about such things, and assigning credit simply wouldn't occur to them. But, I think they would be happy to do so once someone politely alerted them to the issue, and it would probably make them more aware of assigning credit in a similar situation next time.
Funny story: My cousin's wife is a professional photography and posts a lot of pictures on FB. Recently her husband used a photo she'd taken as his cover photo, and happened to crop it in such a way that her watermark was cut off! She called him out on it, in a humorous way, on FB. So even people who live with professionals and completely respect their artistry can make mistakes sometimes.
Re: the cake--I think for me, it would depend on the natural flow of the conversation. IME part of the "this cake is great" conversation is "who baked it," so proper credit would be given naturally. If someone didn't specifically ask, my assumption would be they didn't really care, not that they were erroneously giving credit to me in their minds; and if they said anything about it later--like, "Hey, Lynn2000 makes awesome cakes!"--I would quickly set them right. That's just me and my social circle, though; I can see how miscommunications would arise with people who do things differently, but hopefully we could get everything worked out politely.