Marketing blogger Jonathan Greene describes

• Facebook as a pub crawl (a few friends and some random acquaintances talking about everything)
• Twitter as a speed date (you’ve got a short amount of time to grab someone’s attention)
• LinkedIn as after-work office drinks (a chance to mingle strategically).

Don't boast on LinkedIn. You wouldn't do it in real life, don't do it online. Instead, highlight other people's good work. Talk positively about projects you're involved in but don't amplify your part. People can decide for themselves.

Be true to yourself. Don't boast, but don't undersell yourself either. Be positive and professional, but be personal too.

The reasons you have to be on LinkedIn boils down to you coming across well and being able to back it up in real life.