The Blogging (And Reblogging) Power of Tumblr
Tumblr at one time did not have "no-follow" tags on their external links. Because of this, there was a time you could derive a good chunk of SEO value from well-optimized hyperlink text. Now, this is no longer the case, due to the fact they began to get some serious spammers clogging up the site. So no more of that special “link juice” sauce will be coming from Tumblr. However, Tumblr is still useful.
All you'll need is to sign up for a free account. Be aware that choosing your Tumblr name is important, but it can be changed later if you wish. To get started, search a few basic keywords related to your favorite (or relevant) topics on Tumblr. If you find that there's a good deal of content around those keywords, Tumblr automatically saves those searches as tags that you can continue to search every time you return to the site.
If you have any content that works around those chosen keywords, it can't hurt to post a link to it. Better yet, post a picture or video (YouTube embed is allowed), and tag it with those keywords. You would then want to link it back to related content on your website, not just your homepage! Visual content works best on Tumblr. Text-only links stand a very good chance of simply being scrolled past.
Having trouble finding content around the desired keywords? Then perhaps Tumblr isn’t really for you. Since Tumblr is so hash-tag/keyword driven, you really want to make sure your posts will be getting seen. If you’re not going to get much traffic, it’s probably best to promote using more traditional channels such as Facebook and Twitter.
So what does using Tumblr get you besides free publicity? It can get you a ton of traffic if you post on it often enough with content interesting to Tumblr users. The trick is to get people to "heart" it or re-blog it. Tumblr profiles are search engine indexed as long as you don't make them private, too. All of that content is index-able just as your Facebook fan page and other social media profiles are, except each post is indexed more like a regular blog post!. Tumblr doesn't directly help your own properties’ search engine rankings. But SEO is more than just rankings; it's all about getting found! Also, being mentioned on Tumblr does count at least for something, as even no-follow links still help you in relevancy for the keywords attached to said links.
Reddit: The So-Called “Front Page of the Internet”
As for Reddit, which also needs a free account, their links actually do provide link juice. They do not currently (as of this writing) have "no-follow" tags on their links. So it can't hurt to post links to your content with keyword-rich hyperlinks. Preferably you want to find a "sub-reddit" related to your website content. Do keep in mind that many sub-reddits don't allow commercial content to be posted there, but personal blogs are usually perfectly fine, even if you have affiliates. If you just post your links on plain old reddit (i.e. not a sub-reddit), you may or may not get traffic from it. If you do so sparingly, that link can be useful for SEO purposes. How much that link counts is hard to say, but it can't hurt you.
The primary benefit of Reddit is that it has an up-voting feature. There are also many very active commentators on the site that can do a lot of positive work for you if your content is good. Do be aware, however, that down-voting also exists and there are plenty of opinionated Reddit users. So be careful what you decide to post to Reddit, because there are "trolls" on the site who browse the site simply to antagonize people. Be aware of this downside and if it's not worth the potential upside to you, avoiding Reddit may be a good plan.
Whereas bookmarking tool sites like Delicious and StumbleUpon can be useful to share content in certain niche topics, Tumblr and Reddit relatively large social networks can be a more useful tool in gaining some useful mentions and possibly some extra traffic.