Tumbler is a good portal but i guess using tweeter and Facebook for traffic portals is more good because these have huge activity.... but yes we can use as every activity counts.
I have recently found an interesting article about how to increase traffic to a blog: http://hubpages.com/hub/how-to-drive-tra…
Actually, I am starting to use Tumblr for promoting some of my articles. Back then, I used to ignore Tumblr but recently I found out that it's a good source of stable traffic. Of course, a lot of factors would definitely come into play such as the number of your followers, the quality of your posts, popular tags and hot topics, your choice of niche, etc. Anything is possible as long as you are willing to "research" and "experiment".
I'm uploading picture with my affiliate link on them, and i get lot of targeted traffic. I've shared a bot here to help you out to automate this process Good Luck!