An easy way is to go to local real estate investment clubs and start talking to investors. You do not have to put on a sales pitch, just talk to them and see what they are doing. Over time you will build up a network of clients and will probably meet some very interesting people.
Second, once you build up a network, focus on finding deals. My thought is to learn what makes good rental properties and then go find them. We typically find the house first and then send it out to investors. Our rule of thumb is that if we would buy it for ourselves than it is good enough to send out to our investors.
Once you know what to look for and what areas make good investments, you will have a steady stream of properties to offer and you will be amazed how quickly word will spread.
Start with the deals and the investors will follow.
Once you find some deals that they would be interested in, staying in touch is pretty easy. If you are actively sending them deals every few weeks then you can call or email them.
If you are not actively sending them deals and just want to stay in touch then using Linked In, Google or some other more "professional" type of social media works. Facebook may be fine but a lot of times people use it for personal things and might be reluctant to talk business over it.
Also the monthly investors club meetings are an opportunity just to stay in contact with people. Another thing that you can do is schedule lunch meetings at least a few times every week with different people. The key thing is to be able to offer them something that they are interested in. Listen to what they want, find a way to send them properties that meet their requirements and staying in touch won't be a problem.