IMO Blogs are definitely a milestone in the online world. You can use any tool as you please - I think the important aspect is not WHAT you use but HOW you use it ;-)
There is too much talk about blogs and bloggers - you're a blogger if you have something useful to say and know how to say it.
I find Social networks interesting and fun to use - their usefulness strongly depends on who you are addressing and what your message is - that's my view of things of course :)