I use both Facebook and Twitter on mobile devices. Twitter is something that I don't check often. Facebook is something that I use frequently on devices and my regular computer - both personally and to add to our library's page. While the periodic updates don't bother me much on the computer, I find that they affect my use of mobile apps much more - if I don't like the update, I use that app less until the next "look" is released.
For this thing I decided to open a LinkedIn account. I have gotten LinkedIn requests for a while, so I thought now would be a good time to try. After some exploration and the addition of several people to my network, I can see where this would be a wonderful tool for certain industries. The suggestion of tiered "who you may know" contacts is more accurate than Facebook's suggested friends. I was also surprised that there were people suggested who I knew personally but had no connections with me yet through LinkedIn. However, I am still unsure how much real use I will get out of this app ... generally, if I am looking (as part of work) for someone with expertise in a certain area, I use email or phone to contact people directly.