Database Development

I've been doing a little work with SQL recently, and I've been trying to figure out the best way to develop somewhat of a "Social Networking" database using as few fields and as small of data as I can. I've run into sort of a problem while doing this. I cannot figure out how I would be able to expand the database if this little project took off. Say I start out great, expecting roughly 2,000 users, and I'm very comfortable with that, but lets say that it expands to 200,000 users. My queries would become exponentially slower, and I would have no idea what to do from there.