Is there a reason for this? I have 13 Twitter friends on Tumblr but only 1 FB friend (of the same group)....

Anecdotally, a lot more people I know seem to have their Tumblr's authenticated with Twitter (for tweeting) than Facebook (for sharing on Timeline).

Twitter and Tumblr feel very compatible -- they're both public microblogging services.

Facebook and Tumblr feel not as compatible -- Facebook is private and who you are, whereas Tumblr promotes public posting using pseudonyms.

I especially don't want to connect my NSFW Tumblr to My-Mom-Reads-This Facebook. So not giving Tumblr my Facebook credentials is an easy way to ensure that.

I just tried the Tumblr lookup app, too: http://www.tumblr.com/lookup/

It found 50 blogs based on my Facebook.

It found 1000+ blogs based on my Twitter.

So, consistent with your findings.

Could be a limitation in the Facebook API, or it could be a Tumblr bug.