Marty Blaise

I have a question for those of you who use LinkedIn as sort of a job and office networking website. How long does it or should it usually take for someone to accept your offer for linking to them? My work colleagues usually accept right away, but I find that people who don't really know you don't accept very often. Seems to me you would want to be able to network beyond those you work with daily or so. Any thoughts on this? Thanks.