Linked-In is the de-facto workplace network for me, despite us all being on a new intranet platform, based on Sharepoint.
Don't be misled about sharepoint. There are many bolt-on features which attempt to bring social networking capability into its environment, but they're not native (yet, at least). The datum at the heart of sharepoint's structure is an Office file, not an employee ... great if you want a filing cabinet, frustrating if you want a social network ... despite what the "My Site" features might appear to offer.
Gripes aside, we've found one very powerful way to unleash peer power in our company over OOB Sharepoint functionality: an open business forum, with the most recent x3 thread topics visible above the fold on the homepage, which is becoming the browser's default page in every business. If nothing else, this means that complaints get high visibility and rapid attention from people who can answer/resolve them. In a complex, legacy-laden organisation, that's unlocking a lot of useful feedback.
I have used SharePoint quite a bit. It isn't necessarily easy to set up (I don't do that myself!) but it is very robust and I have found it a great tool for sharing information and collaborating with others.
I'd say a big thumbs-up to sharepoint for getting your business processes online, cutting down on email, paperwork, form-filling, etc, and for doing some broadcast communication ... but make sure you've got 50% of your budget left over for plugging-in and configuring some third party technology, if you really want to get your social piece zinging, especially in the areas of:
* user-driven communities (like this one)
* expertise search
* other forms of socially-intelligent search (eg, people who searched for [your term] rated [this result] )
* mobile users (who need to read human-readable email alerts, not http://gobbledegook )
BTW, have you compared the sharepoint route with rival platforms built around people (not documents) such as Cubeless? Might be interesting ...
Back before we committed to Sharepoint, I was exploring Cubeless, after Corporate Executive Council introduced us to someone involved in SabreTown (running on the Cubeless platform). SabreTown were sharing the impact of their first year of using the platform, and the 'expertise search' functionality alone had repaid their investment.
It offers one of the first applications of a fully-featured "relevance engine" to an intranet. Instead of tracking pieces of 'knowledge', it's powerful at hooking people up, allowing new connections to happen ad hoc. Everyone makes contributions to the intranet, and as the algorithms figure out what you bring to the org, they begin to hook you up with other people you should be connecting with. Give it a spin!
In addition to EEN, I have used LinkedIn and Twitter for professional networking. I find the discussions and "answers" section in LinkedIn to be a great place to share information/ideas/etc (same as EEN). On Twitter, I have frankly met others I probably would not have otherwise met. Additionally, although it is not a professional networking site, my blog has afforded me an opportunity to share with others and communicate with them through comments.