A group of us in my FC just moved to a larger FC... we wanted a more active endgame group, while others in the FC wanted a much more casual vibe.
Something that really helped me out... when you click on a player, you get a list of options such as invite to party, examine, add to friend list, etc... one of the options is "view free company profile." You can click on this to see that FC's slogan, whether they're recruiting, if they have housing, who the leader is and how many people in that FC are currently online. Many FCs also list their Websites in their FC profiles, allowing you to go online and find even more info.
I play in PST, so finding a FC with a strong late-night core was very important to me. After finding a few free companies that looked interesting, I checked profile info from their members over the course of a week or so, giving me a much better idea of which FCs had more stable membership.
I'm now in a great, extremely active FC that does all endgame events, and I cleared T4 on my second day of membership.
Happy hunting!