edit(i forgot to answer your question of where my sites are): I have used this strategy with several sites, dittoeffect.com which currently is over a year old and gets about 1million+ hits a year as well as another site which I will not share the name of which is only a month old competing with some 10 year old credit card sites for some decent terms in the top 10 (3000 a month search volume terms).
Sometimes it may seem that way however the way I see it is that sometimes a tumblr blog is not very popular or does not have a lot of links and page rank going to it so it won't be crawled for quite some time (even up to 60 days sometimes). Because of this you can see a fairly regular stream of links coming in.
Also I doubt you would be as obsessed with it as I was as I was building close to 700 likes a day (700 or more links a day) so I could focus my attention on this over a short period of time.
Being really good with searching on google and through other means I also used this to get links from high page rank sites including 400-800 over PR4-6 page links included in those. The higher page rank pages do get crawled faster though so sometimes it may be wise to avoid getting more than one from a domain. Also since these links are virtually endless, limited by the time and effort put into it, you can just get like 20 a day in a couple minutes and then just forget about it to keep it looking really natural.
Make sure to look at the rules for tumblr as self promotion is not something they support.
A good thing to do though which I don't even really see as self promotion is to search in google:
inurl:tumblr.com "your keywords"
in my opinion by liking relevant pages to your site you are then avoiding self promotion as you would obviously have interest in these pages as well as the added bonus of being able to add 1000s of relevant links.
Another important note is that your blog can be something like ditto-effect.tumblr.com and the links you get show up as ditto-effect. google sees these as links with the anchor text "ditto effect" without the hyphen so thats great too. Only fall back is that these can only link to your main page and only one tumblr account can be pointed to a domain.
Its hard to say the value of some of these links because its not in content though at the very least this is a great way to get page rank to go up as well as get your site crawled like crazy.
If you have any more questions don't hesitate to ask