I've been using, and enjoying Instagram a lot recently. It's currently an iPhone-only photo sharing app, but there's also a web service back-end and the beginnings of an API so that you can connect it to other things. The photos are designed to look like old-school instamatic photos, so they're all square, and there's a range of filters that you can apply. Some people might deride that as a bit cheesy and an analogue fetishism, but overall I think it works.
The most important thing though is that they've managed to make sharing photos really really simple, and as a result it feels far more instant and social than other services. Sharing a photo immediately after taking it means that you actually do end up sharing it, rather than having to remember to do it when next importing them to a computer (which means I usually forget).
The socialness also feels quite personal - it's a nice feeling to be able to visually see the things your friends have been doing and seeing, and the photos can quite easily be a topic of conversation.
I'm not convinced that Instagram is much of a public engagement platform. I don't want to follow brands on there. Or even people I don't really know. For this reason, I recently switched my account to private (feel free to request to follow me if you know me).
Finally, and one of the reasons for this post, is that I've just received an e-mail reminder to renew my Flickr Pro account. The past five years, I've done this without a second thought, but this year, with my Flickr stream being fairly dormant, I'm not entirely sure whether I should bother.