Twitter is how I built my entire network in the Bay Area... it was amazing at flattening hierarchies, I could talk to anyone. Then at some point the recommender system changed and people in my network never saw tweets from one another. At that point it became useless for my career.

Concurrently, LinkedIn sharing grew from nothing to awesome. I get most of my news about the latest developments in my field from LinkedIn. I get jobs by posting blog posts on LinkedIn, or summaries of interesting recent papers.