I see old and full of clients as positive reflections of providers. If their servers are slow, that's an indication of mismanaged overselling which is less than desireable.

What sites did you have hosted with these providers?

A shared web hosting plan is the last one you should think of.Since you are planning to launch a social networking website, you are bound to receive high traffic gradually.But, you cannot predict it as to when your site would receive high traffic. Imagine if your site is hosted on a shared server and there is traffic to pooling in and your sites goes slow, for sure it would affect the visitors, they would simply leave your site and hunt for a faster and better performing website somewhere else. Therefore, it is recommended that, you either opt for a medium ranged VPS with over 512 MB RAM or a starter Dedicated server.

Yes a VPS sounds good for social networking. A regular hosting may have issues with a social network site just because it may get lots of traffic so you may want to look into a VPS.

Yep, a VPS is best at the start and when your social networking grows big, then get a dedicated.

Shared hosting will definitely be unable to support your site.