Like Yuri I have only ever used filkr but find it perfectly adequet for sharing photos.
Ordinary membership is free but limits you to 200 photos when you pass this your earliest photos drop off. You can have a pro account which does not have limit and enables you to get extra stats on views of your photos. Some ISP like BT. This took me some time to crack how to do it.
FLICKR allows you to mark photos as private and share with others by invitation to individuals if you do not know the world seeing your photos.
Also allows you to put photos into different sets, the same photo can appear in more than one set.
You can join groups and share photos, in my view sharing of photos in groups is active but discussions with in groups do not seem that active, TP is the better place for this I think.
Can see no real downside of FLICKR, I use it to link to from TP to show my photos, I keep recognisable photos of people as private, just aware not everyone wants their image in cyberspace.
This is just a personnel view of flick hope someone can comment on strengths or not of tumblr. I have no experience of other upload sites.