Particularly when the startups don’t have a proven business model and are still evolving.
As a consumer I generally don’t like to see my favorite web services getting bought by larger companies. Most often the innovation stagnates when that happens. The recent history is littered with examples of this: skype (ebay) - delicious, flickr, jumpcut, bix, mybloglog, dialpad, webjay(yahoo) - jaiku, jotspot, feedburner, dodgeball, grand central (google) to just name a few. Now I hear that AOL may be regretting its purchase of bebo. I saw that coming when AOL purchased them.
This the reason why I don’t want twitter to be sold to facebook or anyone. I think twitter is onto something and I want to see where that leads to.