1. It is effective as a business electronic rollodex, particularly for casual acquaintances. Makes it easy to find someone if you need something professionally to contact them about at a later date. If you have only worked a few years, it isn't as useful, but put in a decade or two and it is really nice to be able to connect with old friends say, you are visiting their local area and want to get together. It would be particularly useful for networking purposes should you ever find yourself out of a job.
2. It puts you in searches for recruiters - my HR folks use it regularly to find candidates. I am solicited for good jobs on a regular basis through linked in. This is the main reason to be in it.
3. It takes very little time to put in a profile and after that, you really don't have to spend any time on it, unless you want to use it like snapchat, instagram, facebook or even...CB...for your narcissistic needs. This post is primarily meant for those confused folks.
4. Unless you are in sales or similar, needing a whole bunch of professional contacts, I can't imagine ever getting to anywhere close 500 contacts, but then again, I turn down a lot of people that ask to connect to keep the list manageable with key contacts, well, because it isn't facebook. I probably have turned down more than 500. Then again, you never know where a network connection might help you someday. It's like planting seeds, so if I was earlier in my career, maybe that is a mistake to turn down anyone. On the other hand, I never "connect" with anyone I currently work with, unless they have a need for me to make an introduction, say a NBD type person, but do connect when someone leaves or when I leave. I have had 7 jobs and have worked for 35 years and I just don't see how I will ever get to 500. Of course at least 15 of those years were before Linked in was around, so maybe if I was starting now, the number might climb to above 500 after a long time working. It would certainly climb if I connected with every youth I have worked with, or every housewife that gets bored and starts sending out professional contact requests.