I try to add them to my LinkedIn or Facebook. When I move to a new job or accomplish something (e.g. new product, etc) I send an email (bcc). This usually gets the people who want to keep in touch to respond. Then we go back and forth updating each other.
I would rather someone only talk to me when he needs something, rather than sporadically wasting my time by engaging in useless platitudes, when neither of us actually want to.
If every acquaintance "touched base" with me "once-per-X", I would have a limited number of acquantainces.
Don't pretend to be friends with people by bothering them, unless you actually need something, and even then, preferably try to make it mutually beneficial.
If I had a system to organize this, here's what it would be:
1) Add all new professional contacts to a Google Doc along with the date of the last time I spoke to them, with the most recent contact dates at the bottom.
2) Contact one person every week or [insert appropriate time interval].
3) Move the person I just contacted to the bottom of the list.
4) Rinse and repeat.
But instead of doing that I just have Twitter and a dash of Facebook. People pop up in my feed and I interact with them on the fly.
Thinking of taking a stab at this problem. Now taking feedback on www.meetnucleus.com.
I usually send a text or an email once in a while just to ask how they are doing.
LinkedIn.
I'm not on Facebook or any other social networks.
LinkedIn is basically like a Rolodex of friends and colleagues that automatically keeps updated. So it's useful to query, for example, who in your network works at a particular company now. And then you find out that guy from college who you hung out with a few times but otherwise never would have kept directly in touch with, is now a senior manager there.
The regular network updates can be interesting, to see who's gotten promoted or moved on to new positions.
I don't do any of the socially networky things on it, though occasionally read what some people postings.
Regarding keeping in touch, you can contact people via the internal messaging feature, though most of my contacts also have their email address visible for their connections, so I mostly email people as needed.
I'm in the financial industry now, after finishing a PhD in physics. And via LinkedIn, for example, I've been contacted by the dean of the business school at my university (even though I was never involved with the biz school in any way at uni) who was visiting London to invite me to an event he was speaking at. I've had students I don't know from my uni, or friends of friends, reach out to me to ask questions about what it's like moving to finance, advice for getting jobs, etc.