Rebecca,
I live in Ajijic so I'll try to add to others' responses to you. The two webboards are good recommendations, as is Judy King's e-zine (online magazine, $40 US annual subscription) called Mexico Insights (look up the URL--it won't let me post it here.) It's full of invaluable information--especially access to the archived issues.

On to responses to your questions, in no order:
Ajijic is primarily a retirement-aged community. Fewer expats in their 40s and 50s than those 60 and above. There's a whole lot of English spoken here; however, you really limit yourself to the opportunities and life in Mexico if you don't either know--or actively start to learn--Spanish. However, some people do it, especially older folks who find it more difficult to attempt the new language and experience the culture.

It is relatively easy to connect with people here, but you do have to take the initiative. Without work as a common denominator, people's social circles revolve around their areas of interest. Most people are couples; there are many singles, however, but not a good singles network. Lately, I've seen some postings for roommates on the webboards mentioned previously by other posters, although that is pretty new.

Check online for international regulations for pets to Mexico. Shots have to be up-to-date (within the past year, documented) and an International Health Certificate provided within 72 hrs of border crossing (most vets can provide, and maybe even leave date blank for you to fill in.)

It is almost impossible to get an FM-3 with working papers at the moment in the Lakeside area. That doesn't mean that you can't work online, from home, at your US job, but you will be hard-pressed to get a job in the Lake Chapala/Lakeside area. Your US income will be taxed by the US accordingly; get a good tax adviser with international experience to fill you in--before you make any decisions.

The weather Lakeside is marvelous and is a draw for many of the expats, as is its relatively good location for venturing into other parts of Mexico. Temps are fairly constant, except for mid-April thru June; we get several months of heavy rainfall, but mostly at night. Because of the demographics, housing is comparatively expensive for Mexico, especially if you want upscale US amenities. These homes will cost as much--or more--as many parts of the US. Central heating and a/c don't exist. You can rent or buy; most people suggest being here for six months or more before you commit. Neighborhoods differ tremendously and there is no "zoning", as you know it in the US. It really helps to do some serious exploring before putting down roots, but many jump right away.

There are a few well-known movers in the area. They seem to know the ins-and-outs of the international move, but it is very expensive to move goods here--about $1US per pound, or more. Most people bring the minimum and/or some hard-to-find things. Furniture styles from your US residence may be out of place here: we live outdoors most of the year; wood can rot, etc. and your antiques, etc. will disintegrate quickly in the open air/dust. Better to see some homes here before you decide. People can tell you what's hard to get (good linens, towels, tools are limited and more expensive), but you really can get most everything.

Because of the large number of expat seniors and the way the villages are set up, not too many people drive after about 9 pm. That means, many restaurants (none US five-star, or even close, but some decent and trying) close very early. Evening entertainment: music, some dancing, etc. also finishes pretty early. Nearby Guadalajara (about 30-45 minutes) is our airport and hub for box stores, nicer-end restaurants, theater, etc. Many people who go to Guad for evening activities stay the night in the city, as the drive is unlit and the roads at night attract horses and cattles due to the warmth of the pavement.

Lakeside, you can be as busy and active or as lonely and isolated as you wish. People's personalities don't change because they move SoB. However, Mexico is an experience, and the people who seem least satisfied are those who come here, expecting to make changes to the status quo and way of life. It's not the US, nor is it is supposed to be. Successful expats seem to be fairly well-traveled, open-minded, and enjoy the fact that Mexico is not a suburb of the US town from which they came. Those looking for a cheap clone of the US, while maintaining a US-style lifestyle on a shoestring budget, will be greatly disappointed and, to a certain extent, will be isolated. People who have made their home here like it, and aren't afraid to ask the perceived bad apples to vacate the barrel, so to speak.

Lakeside, we have no big box stores, and just three pretty small shops that cater to American import foods and products. Other than the two-lane main drag, streets are 500-year-old cobble-boulder, shared by cars, horses, and pedestrians alike. We have a three-screen cinema and an English-weekly paper from Guad. There's many, many OK restaurants as well as a lot to do, though, especially if you are into volunteering for a variety of charities or are into the cocktail or garden scene. During the winter months (Nov-April) we are inundated by snowbirds, many from Canada, and the ensuing saturation of services and amenities can be stressful.

As to your beach questions, we are about 5 1/2 hours to Puerto Vallarta (called "PV"); about 4+ to Manzanillo. Neither drive is particularly difficult, but there are lonely, very windy, treacherous mountain passages to navigate, especially during the rainy season.

Like any place, it takes a while to make a new place your home. Mexico is not for the faint of heart, but it is an experience that's not to be forgotten. What type of experience it is depends on you and how well you fit into your surroundings. It for sure is not for everyone.