No, people talk too. It's hard to get a conversation going, but people find ways. For example, sometimes they reblog what someone else has written and add their own comments after (it's very clear who said what) and then the original poster reblogs with their response. It just not nearly as easy as LJ.

What I would suggest is finding a couple of Tumbler accounts to lurk around, just to get a feel of the land. Then if you think this might be for you, sign up!