I set the export dialogue in Lr to limit the larger dimension to 1600... but sometimes I change it to 1205 for Lula and forget to change it back...
However tumblr resizes: I just did an up-down and the new size was about 540 x 810. So not the place to go for quality. Probably as well since anyone can right-click download.
I use 500px if I want better quality, but, hmmm... the site is technically flaky as hell. Uploads are done with a Lr plugin, so the resolution is not entirely obvious, but you can look at things full screen and they look ok (when the site is up). They also disable right-click, but there are of course ways around that: if it's on your screen you can copy it, when it comes down to it.
One further downside I forgot for FB: if you set up a separate "artist" page for your photos, you'll rapidly start receiving demands for money. I just use my regular, real-name FB account and only allow access to my friends: that avoids the blackmail, but has fairly obvious disadvantages in regard to who gets to see your photos vs who sees everything else in your life.