This is an overly simplistic statement. They're completely different tools:

Tumblr is a tool for developing (micro?)blogs about any topic under the sun -- therefore identity isn't really relevant (unless you WANT to be an authoritative source on a topic, for which a real identity WILL take you further than a pseudonym on average)

Facebook is about connecting people -- REAL people; therefore pseudonyms take away from that experience. If pseudonyms were in any way positively correlated with the experience, Myspace would still be going strong.

Facebook already proved that a social network where real identity was encouraged could succeed. So saying that Tumblr wouldn't be as popular if it made users use real names would be misleading. I don't think the popularity of Tumblr was because they allowed users to not use their real identities.